I don’t know what to do... HELP!😩😩

Sorry this is long! Please please stick with me!!!

Today I was having lunch with my little sister (she’s 20 and I’m 23) and I learned something that I can’t stop thinking about. She mentioned to me that she was TTC, and I was surprised since she and her boyfriend haven’t been together super long and he doesn’t seem interested in settling down anytime soon. So I asked her about it and it turns out she is TTC without his knowledge. She’s been poking holes in all of their condoms. This is something I am SO against doing, and expressed it to her, and she stormed off. I feel so conflicted because I really think I need to tell him about what she’s doing, but she’s my sister and it may ruin our relationship, and I don’t even know how likely/possible it is to conceive by poking holes. (A little bit of history: she has already been pregnant twice with the same man and both times terminated the pregnancy at his request.)

PLEASE HELP ME I DONT KNOW WHAT TO DO😭