Let's see if I have this straight:
1) Your MIL is turning 90 on the 26th
2) You already had plans to be with MIL on the 26th
3) SIL comes out of no where and starts to dictate that christmas will be held at your mothers on the 26th...?
If this is the case, here is what I would do:
1)Have christmas morning at your house, go to your mothers in the afternoon (like you always have done) and
2)then go to your MIL's 90th birthday on the 26th.
**If SIL wants to have christmas at your mothers on the 26th, let her. It doesn't mean you have to go on the 26th. You already had plans, and forgive me, but God only knows if your MIL will even live to see another birthday, so I think that takes priority.
You can always leave your SIL's gifts at your mothers for them to open the next day...