Plus, I can't imagine hosting a dinner party at my home the night before my son's wedding! Now that sounds like work. Heck, I had most of my kids' parties out, because it was so much easier for me - just write the check, no cleaning before or after, no cooking...
Right. And can you imagine the comments - even well-meaning comments - & how the whole situation could be construed by observers just by various comments made?
"Well, since groom's sister's wedding was in 2 months, poor groom's mother just couldn't do a big rehearsal dinner, so she just had the dinner at her home. Bless her heart."
"Well, we just felt so bad asking poor groom's mother to do more since she was so busy w/ groom's sister's wedding, so we're just having a simple rehearseal dinner & it's fine... it's just fine."
And I don't really mean to be snarky... just depending on various factors, I could see things going this way. There's absolutely NOTHING wrong w/ a simple rehearsal dinner!
OP - You are the bride's family in this situation, &, typically, right or wrong, more consideration
is given to the bride & her family. Be thankful!

I just feel a bit badly for the groom's mother in this situation, don't you?
I have a sister. My husband is one of 4 sons. We now have 1 daughter & 2 sons, &, since I'm now the mother of boys, I'm beginning to be much more sensitive to my mother-in-law & starting to see how, sometimes, the son's family is disregarded while the daughter's family is put first in situations like this.
Again, I realize that often times a wedding date is not going to be convenient for everyone involved, & you have to choose the date that works best for the bride & groom. But I really can see how this could be a tough spot for the groom's family.