If you really care, you dont need to have live-in relation ship to save her.
just give a police complaint under domestic violence act that will save her.
but if you make a live in relation ship with her, after few years she will file rape case on you and make you roam around courts for your entire life.
you dont have to make her as your wife/keep to save her, you can file domestic violence case on her parents if you really care.
if you really care do legally not emotional decision which will spoil both your lives.
take logical and legal decision not emotional decision.