Is it legal for a 13 year old to divorce her family?

So after I posted “screwed up family” things got better, and today is her birthday, but I was talking to her like 5 min ago, and she told me that she was going to divorce our family after spring break! (2 weeks) and she is planning to move in with her friend… She just turned 13! Is this even legal???!?!?!?! I hate becca!!! Its all her fault! If she didnt hangout with becca she wouldnt be doing this!!! She wouldnt have runaway, she wouldnt have hurt herself, and she wouldnt try to divorce our family!!! My parents dont know about this yet and my sis is talking to her friend’s dad and he might say he’ll adopt her (not good). Theyve been looking into it on the internet!!! Ahhh sorry im kind pissed, but you would be to if your lil sis was moving out! Im mean for crying out loud she’s only in 7th grade!!! (im in 9th) and my parents said not to tell anyone about what is happening (running away etc.) with her and I hate trying to be so fake around my friends! I hate it!! I have to put on this act like I have this perfect family (and im not saying im the only one who has the perfect family! There are many others like me) but I cant believe this might actually happen! Ill never get to see her! She would move in with her friend and they might move to ny or ca or somewhere far away!!! First of all I love my lil sis!!! (no matter how annoying she may be) and second I cant live without her even though I dont always act like it! My parents dont know and they think shes getting better, and so did I! This sucks!

So basically my 2 question are:

1: is it legal for a 13 yr. Old to move out (and what do you have to do or can you stop it)

2: I need advice on how to deal with this. Its not just my sis thats going through a tough time… She blamed everything on me! I dont know why! I mean I let her borrow my clothes and do w/e and she treats me like crap but I live and they’re all like ‘poor her’ so yeah sorry I talk to long, I just have a lot on my plate!