Wow! Didn't expect so many responses.
To address some of the questions folks have:
1) Changing the dates is not an option. We are going during spring break.
2) Getting our own place is also not an option. My mother is actually paying for the trip out of the money my dad left her when he passed away. This is the second year we have all gone away together, and we are wanting to make a tradition of it. We could not afford to get a house at this resort! (I'm also trying to keep my mom out of this drama. It seems the hight of ingratitude to create stress for her around this.)
3) I actually suggested the restaurant thing myself! My sister-in-law (I'm crazy about her, btw) does have a "strong" personality. We get along well, because I (and my brother, incidentally) are very easy going. She and I also both have young children, and she's been my rock when I'm about to tear my hair out trying to take care of a colicy newborn.
4) The only kids involved (who eat solids) are their 2 year old and our 9 year old.
5) To explain the dynamic a little more, my husband and I are recently married. We have his son half the time, and spring break every other year. My husband was an older dad, kind of set in his ways. His vacation time with his son is especially precious to him, since he doesn't get to see him all the time. And, for the past 8 years, he used to having 100% control over how that time is spent. Now, he's in a new marriage, has a new baby, and he's really compromising all over the place (and doing generally very well at it) . So, I think this additional request just pushed him over the edge.
Honestly, I just want the vacation to happen. We actually all get along very well, and, with my dad passing away so recently, this family time is important. I think I'll just sit down with my brother and try to work something out.
(Oh, and I didn't mean to post and run. Just had a busy day!)