When I was a MOH, I lucked out because my cousin gave us folders containing specific stuff she wanted us to do. Thankfully it wasn't Bridezilla-ish at all. Most of it consisted of picking up stuff, meeting at a designated time to help make favors, arrange flowers, etc.

In addition to the info OtterXO provided, I'd suggest having a comfortable pair of shoes/flip flops for her wedding day when not in the ceremony and reception. When my cousin got married, she needed this and she needed that. If I had to run around like a madwoman in my heels that day, I would have quit.