Wow, this is really a multi-pronged answer that may take some time
We are also childless by choice, and I have a sister with money problems and jealousy issues, but that doesn't necessarily mean you and I in the same boat. So I'll try to give my advice with a lot of "buts"
First, no way would I take someone elses children, heck, no way would I take another
adult, on our
first trip to Disney. Until you've been, you really can't imagine how different it is from what people usually think of as a vacation. And honestly, because it's so different, people are just as likely to go into it thinking they'll love it and end up finding it's not for them, as they are to go into it being skeptical and find it transforms them. There is no way to know until you go. And it is, in one word, exhausting. It's great (to me), but very very tiring. A first-timer can try to plan like they suggest on here, but unless you've been here before it can often be for naught because you may find you like this but not that and need to change it all, find you hate the commando route after the first day and decide to take it slow, or blow through a park in 4 hours you thought you'd need 2 days to get through.
In short (too late!) there's just SO MUCH to discover your first time, about the parks, yourself and your DH at the parks, that I really believe you should do the first time alone. What if going with four children turns out to be awful and stressful? Or just boring because you're so busy catering to their needs that you don't get to do the things you'd consider fun. Your DH can come away from it thinking, "AGH, WDW, never again!" when really if you'd tried it alone might have loved it. And it may take heaven and earth to get him to try it again. I would go first alone and consider it a litmus test i.e. "Okay, so now I know I love WDW. Let's try it with a kids perspective!"
Also, like many people on the DIS, many of us (even parents!) have been without children and haven't felt overwhelmed by the children in the park. Nor have we felt out of place. And this is from someone who really, really doesn't have the whine/shriek ear deadener that people develop. Like a needle to my brain...
I don't think you need a child to feel a part of WDW.
Now, I'll address the nieces. You don't say if you've traveled with the four of them elsewhere where you've had sole responsibility, but I'll tell you my take on this assuming you haven't. Traveling with four children, even just going out to a theme park nearby with four children, is very very different than having them over to your house and letting them go crazy. I have 6 nephews and nieces, and I can't even believe what it's like trying to take just two siblings at once out somewhere. They get excited, they get overstimulated, and they simply become different kids. At first it's great, they're bouncy and happy, and then they start conflicting with each other and with you because they're all keyed up about doing what they want to do vs what someone else wants to do. And with six people total, there is no way you're all going to want to be doing the same thing at the same time. People (the adults too!) start pouting, start bickering. And eventually someone starts getting tired (or hungry), and real fussiness starts happening. Not to mention, you have to keep your eye in 5 different places at once. I say five, because at Disney (or heck,
Best Buy) husbands have a tendency to wander. There are crowds, distractions, just someone stopping to tie a shoe or take a picture, and it's beyond easy to suddenly find them gone. Add to that trying to corral that many people out of the room in the morning, stopping for bathroom breaks, trying to wrangle five people to eat (and agreeing on what to eat) and your actual time enjoying the parks just slips away. I find myself flustered just trying to order for 4 people and get them seated and ready to eat at ChickFilA. It becomes a huge production, can't imagine 3 meals a day at a theme park.
NOW, most of the people on these boards have kids and do it all the time. But you aren't used to it, and now you're going to go into it with a WDW trial-by-fire of not just one but four children (and siblings!). I'm not saying it's something you should never do. What I'm saying is that it would be much easier if you do this after you've already been to WDW, and find that moving around is easier and more comfortable (plus you've done the things YOU want to do, so not such a bummer if you're restricted by what the kids want to do).
Whew, now all that being said, I'll address your sister. You say she's jealous, but you don't say what kind of jealous. Is it simply something you perceive, kind of a sadness jealousy? Or is she snarky-jealous, with snide little comments? Sadness jealousy is much harder to handle -- she's just being human and you have compassion for her. But snarky-jealousy doesn't fly with me personally. My sister is snarky-jealous. When her daughter asked her why we get to go to WDW but they don't, she curled her lip and snapped, "Because they're rich!" I was furious because that is so very very far from the case. Yes my sister is on a limited budget, but she is also terrible with her money/possessions. Eats out every night, but struggles to pay rent; borrows money but doesn't pay it back even when she can; gets over $5K+ a year on tax returns, but rather than pay down her debt or put it into savings, buys electronics and a new wardrobe. But she is more comfortable thinking we are just lucky or spoiled rather than admit that we make responsible choices. With that kind of jealousy, I have no patience. I hope that is not the type you are dealing with, but if you are, her jealousy is not your problem. It is hers and you can't do anything about that.
Either way, while it's very sweet that you are so worried about her feelings, I can only imagine how she would feel if she knew you were hesitant to approach her for this reason. If she's snarky jealous, she's going to take it as you being superior acting (martyr). If she's sad-jealous, she's going to take it as though it's pity, and I can only imagine she wouldn't want either of those things. Again, you're obviously a caring sister, but as long as you're not behaving in a boorish, "in your face" way on a regular basis about what you have, you really can't shoulder her feelings this way. I'm sure she loves you and wouldn't WANT you to. I'm sure she's also a very loving mother, and would be hurt if she thought someone would think she'd put her pride above giving her children a great opportunity. If you really want to do this for them, you just have to move forward and trust her to be an adult about it. Otherwise, it's not fair to your nieces, to her or to you. But I appreciate the toughness of your situation, so sending you this:
I will add, finally, that having my sister, I wouldn't take her along on a trip with her kids. Believe it or not, she'd make the trip 10x harder than the kids. But in general, even if your sister is not remotely like mine, if she already has some jealousy issues, it might really be humiliating to have her "rich" sister paying her way. A parent doing that is one thing, but your sibling... Then add in the marital issues and you've got a stranger situation. If they aren't already separated, only inviting your sister and the kids would seem to send a signal that your BIL is not a part of the family. But inviting him would be bringing along a bad/tense situation to what should be a happy event. Just something to consider.
Whew, okay, I think I've addressed everything I wanted to. Again, it was colored by my personal experiences, just as is the other advice you've received. Only you know your sister, nieces, yourself and your DH. I'm sure you'll make the best decision. Best of luck!
