First of all, you're the only adult among the three of you sisters in this whole thing, so any legal action would have to be initiated by you... but your half-sisters are still minors, they live under your dad and your stepmother's roof and thus, legally, your half-sisters generally have to abide by their rules... unless you can prove mistreatment of your half-sisters- by, at least for one, your own father!- in court, there is very little you can really do on the legal side of things...
outside of the legal system: family counseling? your half-sisters (especially the older one, at 14) somehow showing their parents that you are not a threat to your dad and stepmother's household and also that they can be trusted in continuing their relationship with you?... I wish you best of luck!
I don't know where you live, but in most English-speaking countries, a legal separation means that your parents are "officially" separated in the eyes of the law.
The usual reason for legal separation is that in some countries/areas, a couple must be "legally" separated for a certain time (2 years where I live) before being allowed to divorce. Your parents may get a divorce later on or they may not. Also, the second commenter is right. Legal separation also means that your parents don't have to be responsible for each other's bills if one gets into debt.
To get a legal separation, your parents would have to provide some proof that they are separated, like a letter signed by both of them or rent bills from two different houses.
Sorry that you're having to deal with this! You should feel totally comfortable with asking your parents anything that confuses you.