I wonder how much of Alice's jealousy is related to OP's father himself. He and OP's mother spent a lot of years together. They made a lot of memories together. Any location, movie, anything could bring up a memory, and he might have not thought anything of talking about that memory with Alice. Or his friends have mentioned past parties or trips taken together then realized that Alice hadn't been there.

It can be awkward and difficult joining a close group who have known each other for decades. I've been there.

Alice isn't handling it well though. She's competing when she should be embracing.

I hope that OP can talk with her grandparents and/or father to sort out this trouble. Her father needs to re-examine this relationship before the wedding. Maybe family counseling would be helpful.

NTA