S.S.
If you are the one that is going to try to "talk to her" don't do it. That is the responsibility of the mom not you. My daughter is 2 and her father saw her only once at 3 months. I constantly worry about what I will say to her when she starts to ask about her dad but that is my right and responsibility and no others. Even if you think your sister is not a perfect mom please trust her enough to deal with this sensitive issue. I am sure that she loves her daughter alot and wants what is best for her and is also worried about how to respond so if you want to do anything talk to the mom first.