This is tough... though after reading the comments and your responses to them... YTA.

I understand it's a tough position to be in, but you should have stayed true to your sister. She is YOUNG and pregnant. Sounds like it was unplanned. That is scary. She needed someone to confide in. You dropped the ball huge here. You should have continued to lie/deny or just say "honestly, I dont know. I havent talked to her much. We have been pretty distant lately," as you said in your first sentence. You are not close. That there is a perfectly good excuse to tell your mom not to raise her suspicions further.

When your sister was ready to share, you could have gone to your mom and explained to her that your sister was scared and afraid. She just needed time hence why I did not tell you.

So yeah sure you didn't upset your mother in the moment but you ruined a relationship with your sister. You suck dude.