[feelingreyt]

It would not hurt for you to call your sister's parents and discuss possible visits with them. You could ask them how she handled meeting your mom.
Your sister is young and it may be emotionally taxing on her right now.
As for telling your mother you had a visit with your sister: What they don't know, won't hurt them. I don't think it would be dishonest not to tell them, UNLESS they ask. If they do ask, just be honest and tell them why you are allowed visits and why they are not allowed visits(if that is the case).
Good luck!

By the way, my son has 3 biosiblings that we keep in contact with(they are not in their mom's custody). They are all very young, but I think it's important for them to know each other.