Lunch with the girls in the bridal party if people don't know each other that well is nice. Something with the mum and MIL and any close female family. Pamper day sis a great idea, some groups will come to a house with one or two therapists and you take it turns to get treatments, while one is having a facial the rest of you can sip bubbly or your poison, eat nibbles and chat

Lovely you can be involved in your sisters wedding and be there to help her out.