Am I being a bitch?

okay so basically over this weekend I learned that my sister cheated on her fiancé... I know it's the worst thing possible but backstory. My sister and her fiancé have been fighting for months and a couple weeks back she came to me and told me she wanted to leave him because she was unhappy but this weekend come to find out he found her cheating I don't know what the extent was but he found messages. I don't want to ask my sister because she's playing dumb but I feel weird now I don't know how to act around her because while it does not affects me it did change the picture/idea of her I had. I do not tolerate cheating at all so I feel weird around her because first she lied to be and continues to lie to me about the relationship which is fine but I wish she wouldn't have told me nothing instead than making me feel stupid. Should I just move past it and push my feelings down or am I ok to feel weird ?