If they observe Passover rather strictly, I'm surprised they would even stay for those two days; if I'm reading right, the last two days of the vacation are the first two days of Passover. We have friends who are strict Orthodox Jews and they completely scrub their kitchens, get rid of anything with a "rising additive" in it, and use different dishes. To them, it's not just a case of no bread or pasta; there are special meals for the first two nights, and ceremonies to go along with the specific dishes.
Out of consideration, I'd manage to do without bread for a couple of days if they are going to forego their other traditions for a family vacation. If your SIL wants to cook the ceremonial meals, I'd let her; some of that stuff is really, really good and hard to make.
But meals for this are very likely to NOT be vegetarian.
And I agree that if the SIL is really doing this right, it's going to be more than just not eating bread or pasta. She would be bringing her own set of pots and pans and dishes, silverware, etc, it's going to be a BIG deal.
And IMO if she's not doing all that, then this isn't that important to her.
Maybe you could make a big pot of vegetarian chili for the last 2 days for your family.
If rice pasta is OK in a kosher household, maybe you could use that? We use that because of DH's Weight Watchers and I can't tell the difference between it and "regular" pasta.
Just an FYI, corn and rice are not kosher for Passover.
Good to know! I had a feeling there was going to be a long list of things that are vegetarian but NOT kosher for Passover.
Items which seem acceptable for Passover but may not be:
* Soda: Most sodas contain corn syrup. Since eating corn is a no-no, soda containing corn syrup is also out. Even if corn syrup is not used, sodas generally have "additional flavorings" which are not divulged and could be derived from grains. Only sodas produced under supervision of a rabbi or other official certified agencies are acceptable.
* Marshmallows: Not allowed unless made under supervision. They contain gelatin, which is made from the bones of potentially non-kosher animals.
And marshmallows absolutely always contain corn syrup. So if corn is not kosher for Passover, marshmallows will never be.
And of course they aren't vegetarian for the others in the house.
All items that are kosher and kosher for passover say so. Coke (reg) is made kosher for passover..and is wonderful since it has to be made with sugar

.
Hard to find, though!
Oh if I had a nickle for everytime someone said something like this to me!

People are just not very respectful of a vegetarian lifestyle, unfortunately. For many of us, it is like a religion because it is that important.
My opinion? This is not about food or religion or anything like this. I think it is about people who are not comfortable enough with each other to share a beach house. I think other things will crop up during the vacation and this is one of those trips that you will wish you didn't take.
I agree (especially b/c for hubby it IS partially for religion, in that he is Buddhist and doesn't want to kill any being...I'm not Buddhist but don't want to kill things and there is a whole long list of reasons I've been veggie since I was 20).
And I agree!
This is not childish response vs unchildish response. If the SIL is ONLY keeping bread/pasta out of the house, that's not really honoring Passover. So she's just putting some little part of it onto a family who chooses to eat in a certain way for, I'm sure, very good reasons. Any long-time vegetarian who accidentally eats meat can tell you, it HURTS to eat meat when you haven't for a long time!
You guys need to really talk some more. But ultimately I think you're going to have to change the dates so this time of year doesn't come between you, or rent more houses.
To find things that are vegetarian and kosher for passover isn't going to be something that's necessarily easy to do, and for SIL to ask this of your family is a big deal.