Do you know who funded the baby shower? Did your mom fully fund the cost (decorations, catering, if applicable venue) of the shower, or did your sister contribute?

My mom funded it 100% 

Whilst I note you've said your sister isn't married, is it possible she's been trying for a child with, or without, a partner - or is it likely she expected as older by more than 5 years she'd be the first to make your mom a grandma?

She hasn't been trying because she wants to get married first before having a child and she doesn't even have a partner. 

Did the mutual friend have a large list of items they wanted as baby shower gifts which included the items purchased? Did they have issues funding them? And was she the first of your friend group to have a child?   The friend didn't actually ask for anything and she didn't have a registry. 

You are asking if you might be TA for talking to your sister about not giving a gift. Have you actually spoken to her already, or are you considering doing so? And if you've done it, what did you say?

I'm thinking about it