You should listen to this podcast -->
http://www.disunplugged.com/2016/01/25/dis-unplugged-podcast-012416-disneyland-show/
It's all about going to
Disneyland with little ones who are in the "free years"...aka under age 3, so admission is free.
I think that you should bring the 2 year old. Given your SIL's refusal to watch your 2 yr old for a few min while you go on a ride, you should probably assume that SIL & her family might want to venture off on their own for part of the day when you are at DL/DCA. And if that happens, it's TOTALLY ok. Your family...even if the trip is just with your 5 yr old...will probably be touring the parks at a different speed than your SIL's family. So just give yourself permission to have SIL's crew wander off and do their own thing for a couple of hours. It will likely make for a more enjoyable trip for everyone.
Will it be easier to leave the 2 yr old with Grandma? Of course it will. But you should also expect your 5 yr old to talk non-stop about DL to your 2 yr old when you return from the trip.
Since your SIL & her family won't watch 1 of your kids so you & DH can go on a ride together, you'll need to use Rider Swap. It looks like they officially called it Rider Switch Service now.
https://disneyland.disney.go.com/guest-services/rider-switch/ We've used it before and really liked it.
Instead of looking at the Rider Switch as a negative (i.e., "spouse and I won't be able to ride together!"), you could actually look at it as a positive. For example, it's hard sometimes when you have 2 kids who are 5 and 2 to have special moments with
just mommy or daddy, where the child doesn't have to share Mom or Dad with his/her sibling. This is what you can use Rider Switch times for. My kids are 8 and 10 and we still do the "go on a ride with Mommy or Daddy" thing and it's 1 of our kids' highlights of the trip.
With your kids being 5 and 2, they are both still in that magical age where all of the characters at DL are the real deal. Once kids hit age 7, they start to have doubts...although my 8 yr old YDD still believes. It's really wonderful and there's nothing quite like it. You'll have plenty of years to go on rides with just you and your spouse. But your kids are in the "magic years" for just a brief amount of time. I think that you should take advantage of it.