Not to want to suprise 90yr old for birthday.

If I were the 90 year old, I'd soon need a lie down in a quiet place!

This is a difficult one. I personally detest any sort of surprise party so my instinct is to say the surprise element is a bad idea. But, and it’s a big but, your sister in law has been asked/allowed to arrange a celebration. It would be disrespectful to simply override her plans so on balance I think the surprise element needs to remain.

Could you speak to your sister in law about your daughter and ask how she would feel if your daughter visited her grandmother as part of a ‘normal’ visit and under strict instruction not to let the party cat out of the bag? Otherwise it sounds as if your daughter has plenty to occupy her before the party. Whatever the decision I hope the birthday celebrations go well.

whose benefit is this event really for.
the aged GM, or to stroke the ego of the go-getter type A person?
90 yr olds should not be surprised, it may be too much for her. it can be difficult for a younger, highly organised person to put themselves into the position of someone who is less powerful, due to age.
your D sounds more in touch with GMs real needs and wishes. she should do what she deems best for GM.
could she tip her off quietly, GM, so she is prepared, not overwhelmed by all these people suddenly appearing, however much she values family.
would GM be able to not let Type A know than she has been fore-warned. does your D have GMs confidence.
good luck.

It’s Canadiangrans MIL not her mother.

Canadiangran. I’m on the fence with what is best. Out of curiosity why is SIL not yourself or your brother not organising your mother’s 90th do?
It seems like perhaps between the lines you’re not a huge fan of SIL, so I’d try leave that at the door and focus solely on what your mother would want.
I just organised a birthday for my MIL in her 90’s, and I know she is a very anxious woman and wouldn’t like a surprise but tbh she was such a nightmare leading up to her birthday I think it would have been easier on her and me had it been a surprise.
If she is generally fairly lively for her age and social I would leave the surprise aspect in place, she will have such boasting rights afterwards with her friends and acquaintances.

Your sister in law has organised this for her mother; you have to abide by her plans. Once people start interfering with reorganising carefully thought out plans, trouble ensues.

Can Your daughter go to see her without letting on about the other plans.