Family troubles with partners mother.

Okay so ever since me and my partner had gotten together his mother did nothing but bad mouth me, she's like (43-46) years old, I helped her, gave her money when I was 14. I was always kind to her and with her children (partners siblings) I'd help them with anything. Cleaned this disgusting house with my partner & invited them over foor dinner but they never came except his sibling.. But his mother was always rude to me. My partner decided he wanted to move in with me and my nan when I was (15) because his mother was saying he had to work and pay her ($400) dollars board and we were still in school. 
She's been saying I've taken her son away, even though when he ran away from home when she had a  (new bf) over, she didn't care that he was homeless. She wouldn't let him back in the house. 
When we were 16, he got our own house and quit school and worked, he became a boilmaker, as I became a worker in the cafe full time , then became manager. We worked and we lived life. We traveled around Austrlia and around the world, 
And all this the my partner mother never contacted us, all replied to us or called us back. 
17 I fell pregnant with my son whose nearly 2 in couple months time. & I've let her in my sons life couple times but during those times, she had been rude, disrespectful and really unapropriate. I have given her multiple chances to fix up her act but she gets worse and worse. We all think she has a sickness as she takes herself to the hospital and lies about everything to make everyone feel sorry for her, she brings up her past a lot when it doesn't matter and nothing to do with us, and always threatens me. I'm 19 now and on baby #2 and I fair things are only going to get worse. Please give me advice to deal with tbis, I have tried talking to her asking her nicely to leave us alone, but she just rights bad post on social media about me. I am trying to avoid court as I don't have 5k to pay to a latest then another 3k for the case.  Want any advice, I want a happy life. We can't move as we have great jobs here and this is our kids home and all. So if you can give me friendly advice to this situation I would very much appreciate it all, sorry it's so long.