Should I stay or Should I go from my marriage

I (30f) have been married to my husband (40M) for 8 years now. We lived well and are simple people. He brought his 23 year old niece to live with us. It's been close to two years now and her behavior is unbearable. She is rude and disrespectful and believes I am the reason her uncle, my husband, is mad at her. In reality it's the fact that she has been living rent and bill free and never has money. That is why he is mad. I talked to him but always says that he will not kick her out that I just need to ignore her. I feel he puts me to the side like what I say doesn't matter. For him it's blood is thicker than water kind of thing. We talked and it seems he doesn't consider me family, but added on to his existing family if that makes sense. I have pretty much given up. I am getting ready to just move on. He can keep his family. I think what really hurts me is that we had a child that died at birth. Maybe the idea of having children is what makes a family to him. I'm pretty much done, but I self doubt. Does anyone have any advice or outside perspective. Maybe I am wrong.