How does one stand up for themselves and still be respectful?

My boyfriend and his mother take advantage of me... Throwing my things away, taking things that belong to me, his mother always just walks in (Never knocks)! It doesn't matter if your watching TV or not she'll grab the remote and change it to whatever she wants to watch! Both of them gossip about me cause I'm not the best Suzy homemaker!! I feel that I can't say anything to either of them... I live with my boyfriend, he pays all the bills and I supply the house with all the food, BUT his mother owns the trailer that we rent! I don't know how to let them know that this is unacceptable to me while still being respectful and not crossing any lines/boundaries
How does one stand up for themselves and still be respectful?
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