How can I be there for my little sister so she doesn't feel the need to look to boys for acceptance and approval?

About a week ago Child Protection Services stepped in when my mom and her bf went to jail for meth. Again. The state put my little brother (11) and little sister (13) in my (27 m) care for about a year to see if mom can step up and prove herself. Both our mom and the kids dad are giant pieces of ****. Parents always fighting, doing drugs around the kids, talking to the kids like they are garbage, making them go hungry so they can get high, neglect, emotional abuse and all the other stuff that goes along with that. My brother is very angry but I went through that too and I'm helping him cope and learn. I got both of them phones so I can keep in touch with them because there's a lot of family helping to pick them up from school and stuff so I can't always be there.
Well come to find out my sister has an unhealthy obsession with boys. I was using her phone yesterday to talk with tech support and get mine fixed and a barrage of texts were coming through. She's talking with one in particular and they are pretty deep into a conversation about who they are and what they like to the point that she is asking for some of his favorite foods. Her conversation with her bestie is her obsessing over one boy or another while her bestie (15f) is just saying "ok yeah that's cool".
I think she wasn't getting what she needed from her dad at home and is turning to boys to try and get it. How can I step up to plate and give her what she needs so she doesn't end up 16 and pregnant or something. It's one thing to like boys at 13 but her whole day apparently revolves around what boys she likes and trying to win their approval. If you guys have any advice I would be forever in your debt.
( I would ask my dad but he dipped on my other sister and I about 18 years ago.)


Source.

So....13 y/o girls are interested in boys? Shocked I tell ya, shocked