Dont over react with the situation. it may became worse if any sudden reaction taken aganist her. make a planning to record the telephonic conversation of her with her boyfriend, take lots of photographs of both of them and arrange as much as proofs as u can. record the threats from her family. which can help you.
DONT MAKE THE MISTAKE OF BUYING A FLAT IN HER NAME, NOT EVEN IN YOUR NAME. IF POSSIBLE BUY THE FLAT IN YOUR MOTHERS NAME.
DONT DISCLOSE YOUR BANK ACCOUNT DETAILS TO HER. IF POSSIBLE MAKE A SECRET ACCOUNT AND KEEP IT WITH SOME OF YOUR FRIEND (TO WHOM U TRUST MORE)