The OP says she later found out that her boyfriend's mother insisted she be in the wedding. That makes me wonder if the bride asked the OP to be in the wedding. I can see it being a bit unusual for somebody you barely know to ask you to be in the wedding party, but the OP may have saw it as the chance to get close to her boyfriend's family and/or thought participating in a wedding would be fun even if she was a bit reluctant.
My question is who actually asked OP to be in the wedding, SIL or FMIL? If the bride asked OP to be in the wedding and treated her like garbage because she was resentful of that decision, that was very rude and nasty. If her FMIL was pressuring her to include OP, she should have either stood up to FMIL and not include OP or treated OP kindly if she decided to make her part of the wedding party. There is no excuse to treat the OP badly in that situation. If FMIL told OP that she was part of the wedding party, I have a bit more sympathy for the bride, although I still think the way she treated OP was unfair and nasty. If this was the case, the bride should have gently explained the situation to OP and let her know that FMIL made a mistake and she was not in the wedding party (or just treated OP kindly if she didn't want to confront her).
I also think FMIL is rude and her pushing for her own way during the her son and FDIL's wedding was the catalyst to OP's bad relationship with SIL. Why would she insist the bride have his brother's girlfriend as one of her attendants? I can't imagine the bride would have included you unless FMIL put extreme pressure on her to do so. It was terribly unfair for her to put the bride in that position. Did she know how the bride was treating you? Why did she insist you be in this wedding, but not pony up for the expenses? An expensive bridesmaid dress is a lot of money for anybody, but especially 21 year olds, many of which don't have full time jobs.
I can see the bride not wanting a stranger in most of the pictures. I also don't think she was under obligation to pay for OP's dress, even if she paid for the others. However, I can see why the OP felt left out and unwelcome. I still the bride should have either accepted OP as part of the bridal party and included her or she should have polished her spine and said 'no' to her being in the wedding party. She took the easy way out by appeasing her FMIL, yet treating the OP like dirt. From the details of this story, SIL does not sound like she is a kind person.