She should send him parking. If he won't be reasonable enough to move out then I'll suggest violence should be used rob push him out.

Anonymous 2, you should not even think of building the feelings you already have for you girlfriend's younger sister. It's something you shouldn't even consider.

Anonymous 3, marrying a lady who has been of so much help may not be the only way to show gratitude. You should discuss it with her (she may not like the sound of it though) and let her know how you feel.

Promise and make sure you are there for her whenever she needs you.
In truth, staying in the relationship out of fear of been considered an ungrateful person may not benefit either of the parties involved.