I just found out my 14 year old sister lost her virginity. 😭😭

I'm almost 20 and my 14 year old sister just told me she lost her virginity. She lost it to some random guy who she wasn't even dating. Im so glad she came to me and trusted me enough to tell me but at the same time I'm so disappointed. I still think of her as a baby😭. She's not going to tell my mom because she will completely flip (she's very overprotective) but she wants me to get her the pill. I told her I would but I feel like I'm betraying my mom. The teenager in me wants to help my sister but the adult in me wants me to tell my mom. 

Haaaalllppp.