My in-laws are so difficult to deal with! And now it is causing problems in my marriage.?

My problems with my mil and fail all stem form issues with my sister in law. From the time I met my husband in high school she has always been given priority over their other children. Anything she wanted she got, anything she did wrong was not her fault, whenever she made a mistake...ex) getting 2 cars repo-ed they bought her one with their own money. She is the oldest of all their children 25 or 26 now and they continue to do this crap. When my husband was 18 they immediately Told him that he needed to get off their auto insurance. But they continue to pay for hers. When she got caught with a bong under her bed....she simply said it wasn't hers and that was the end of it! Our real problems started when she got pregnant a while back, she lies so much no one would even believe her until they saw for themselves, right away she moved back in with them and quit her job, feeding them all this bull about how the father tricked her into sleeping with him and it wasn't her fault. During the pregnancy she had so many opportunities to get things for the baby for free...but she had to have the best of everything which they bought her. She ended up suckering the father of the kid into taking her back and her parents moved her 8 states away to be with him and gave them all their living room furniture. They paid for her plane tickets back and forth, bought her a new digital camera which they claimed was "for the baby." while she was pregnant we got into an argument which I am totally willing to admit was our fault. We m made our feelings about the situation known and. Informed the father to get a paternity test....even after apologizing she never drops it....anytime she wasn't money she will say if we give it to her...she will "forget the whole thing". Finally things came to a head when my husband got in a car wreck will fighting with her on the phone....from that day on I said I was done with her and so was my husband....but his parents keep pressuring us to spend time with her and her kid. My husband will never stand up to his parents about they way they treat her...so I just have All this built up anger against them all. We got married almost 2 years ago and now my sill is engaged to the. Kids father. When we got married his parents would barely pay for the rehearsal dinner, they even asked everyone not to get drinks other than water. And they got us absolutely nothing for a wedding gift. Now that his sister is getting married they are paying a florist thousands of dollars to deliver the flowers four hours away to another city because THAT is the florist she likes.....idc about the gifts or the obey....it is the principle that makes me mad. I just want me and my husband to be treated fairly. Since my husband will do nothing I feel like I have to be mad for both of us and I really don't think I want to bring children into a family where ii know they will never be treated equAlly. I don't know what to do. I love my husband, but I can't just keep pretending like there isn't a problem.