Legal Question in Family Law in Texas

Technically in Texas for civil purposes she is not an adult until she turns 18.

So the moment she turns 18 she can move out of the house.

I suggest you call your local policing agencies - police, sheriff and constable and see their current policies regarding 17 year olds. They might be different in each office.

I doubt that CPS will get involved with a 17 year old since they are overwhelmed with cases. Again, you might want to call their local office and ask their policy.

You don't state who the primary custodian of your sister is currently. This is the person who controls. If your sister would like her custodian to be changed and everyone agrees, it is a simple legal procedure to do when everyone agrees to sign the paperwork. You would need to retain a local attorney and have them draw up the change in paperwork. It can be done quickly since there is no waiting period for a change in custody. But I doubt that the boyfriend would qualify to be the new primary custodian of your sister -- if everyone agrees then you could ask the judge to appoint him.