My brother's then-future in-laws told my parents they would pay for the rehearsal dinner, too. My mom was an old hippie, and while she believed in marriage (later in life religion took over), she didn't believe in *weddings*. Had she survived to my wedding, she would have made an incredible cake for me, but I would have had NOTHING of the wedding that I had, b/c my dad offered to pay for it (which wouldn't have been done or accepted if my mom were still around), b/c a Wedding wasn't important to her, and she had nothing set aside for such an event.
Therefore, they obviously had nothing set aside for a rehearsal dinner for their son.
The in laws have only girls, and the mom never had her own poofy wedding, so was living it all through her daughter (the older, second-to-marry, daughter eloped, LOL, to escape the madness she saw with her little sister). But my parents saw no reason for this woman's madness to change her life all that much.
In the end, my mom and stepdad did host a casual dinner at a local, good, Mexican restaurant, that was truly appreciated by all except perhaps the other parents.
Like I mentioned, my dad did pay for my wedding (offered when I told him I was getting married, and I figured it was a good way for him to pay all that back child-support he didn't have when I was a minor
, so I said "OK"). We did ask hubby's parents, and while MIL was rather opposed to spending the money (and she's from another culture where this sort of thing isn't really done...though to be honest I was TRYING to have a Korean Buddhist wedding and she would not help me!), FIL insisted. We had a dinner at a nationwide chain, Spaghetti Factory, in a back room. It was good food (I like their food for what it is), we were away from the other patrons, it was fun!
Now all that said. WHY are you her MOH? Do you support their marriage? Do you support your brother marrying her? Do you anticipate divorce proceedings inside of 5 years? I will ONLY stand up for people whose wedding I believe in and support, and I made sure that our wedding party consisted only of people that supported our marriage and us together, not just "well I hate that person but I'll support YOU". If my brother asked me to be a MOH for a woman I don't like, I would tell him that it would be more appropriate for me to be a Best Woman or Groomswoman, and then I would very likely tell him that I couldn't do even that, if I disliked his bride that much.
Might be difficult to do at this late date, but man oh man...being in wedding parties can be hard enough, let alone when you don't like each other!