Personally I would thank my sister at the shower make a little toast or whatever but save the gift and thankyou card for mom after. Your sister did nothing so she doesn't deserve a gift for all the work. We don't know your sister though, maybenbsp;she is typically like this, in which case maybe it's the thought that counts and you should have seen this coming?
Nope this was really unexpected. She is really good for parties and get togethers. So when I found out, I was really surprised. Especially when I found out she tried to move it.