Some people have unhealthy boundaries just like the asker. She wanted to help them out for a little while, but found herself on the shit end of "helping out" your family and did it longer than she wanted. She obviously wanted a good relationship with them, but where did that go in the end? The same result if she had not rented below the desired rental price. They knew she was a pushover and took full advantage. 1.2k in a 1.9k area? Talk about family discount!
Why not increase the rent? Why'd let the niece's mother move in? Still "helping out"? The goal was investment property and should have kept it strictly that way. Only person the asker should be angry with is herself for being a pushover for 10 years. Too many opportunities to change the rental contract and she blew it.
It is a hard pill to swallow, but separate out business and emotions. It doesn't mean 100%, but you have goals and there's no need to have people holding you back especially when they owe you nothing. Grow up.