Adopting my sister’s baby?

So I have a 14 year old sister who is very flirty and is getting boy’s attention. My parents don’t talk about sex due to religious reasons. When the time came for me I had to figure everything out myself. But my sister has some mental problems and severe anger issues. I have this gut feeling that she is going to get pregnant and I’ve decided that I would adopt the baby if that happens, if that’s what she wants. My parents are in their 60’s and not financially stable. I’m willing to take on this responsibility if needed. Legally, can you adopt your sister’s child? I know this sounds crazy haha but I’m just curious

Edit: I’m 20 years old

I really should talk to her about sex haha, I somehow never thought of it. I was raised to be embarrassed by it but that won’t do her any good