What should i do...?

My younger sister (she's 16) has sex. She doesn't talk about it openly, but she doesn't deny it or hide it. When my mom realised, she was really pissed, for some random reason. I don't know why, I was pissed at my mom for being pissed at my sister when she'd done nothing wrong. She was in a long term relationship. So what was the big deal? I don't understand why parents are so fussed about kids sex lives. I haven't had sex, so my mom uses me as an example to my younger sister. I was talking to my mom about this and some choice words were used (she ended up calling my sister a whore) what should i do?