Living relationship

First for most you can go in live-in relationship when you and your partner both attain the age of 18+ then you both are entitled to stay together in live-in relation.

If you have fear that her parents may file case or complain against you then i would suggest you and your partner to have a live in agreement in which you should mention all the ground example consent, expenses, date when you started live relation, etc.

And then if her parents file a complain you can show that agreement to police showing that the partner who is residing with you is residing on her own free consent and not under any compulsion or undue influence.