Should i go or should i stay?

Hi all, I'm struggling with something and I hope someone out there might be able to give me some insight on navigating the problem.

My brother and his fiance are getting married this fall (2019). A year and a half ago they asked me and my two children to be in the wedding party. Which at the time I was honored to accept. As time progressed I started having problems with my soon to be sister in law. She would call me in tears because things weren't working out the way she wanted, or quick enough. I would listen and offer suggestions, but then she would get mad at me and ignore me for months at a time. When it was time to pick out the dresses, she told me that as long as it was a certain color and floor length then the rest was up to me. I tried on about 10 dresses before she decided on one that she thought was acceptable. It was my least favorite and the most expensive. My 3 year old daughter was to wear pink, and again after many dresses, she picked the most expensive. Our two dresses combined cost me $600 CAD. Shortly afterwards, she randomly stopped speaking to me for six months. I tried reaching out to her, but she would ignore me or I would get short answers from her. Her bridal shower is coming up, so I've recently heard from her and her mother. I was instructed what to wear, and she requested that I have my outfit and my daughter's outfit "approved" because she wanted to make sure no one was planning on wearing white. Her mother harasses me, and is constantly demanding that I pay more money. They have planned an extravagant shower (without me), and I was told that I would be receiving a bill for my share. When I asked for a heads up on the cost, I was initially ignored, but her mom has told all the other girls and the bride that I dug my heels in over money and now anytime I speak with them, they're rude to me and make snippy comments like "don't worry, you don't have to pay for it" All I asked for was an amount because I don't like surprises.

Her shower has cost me $350 and it's not even here yet. I have to travel 5 hours with two small children, make a ton food and spend two days decorating. I had planned on taking my son to have his tux fitted, but the entire two days has been completely used up. Despite that, I was told that while I'm in the area, I really should take him in.

The bachelorette is at the end of August, and they have arranged a 3 day stay 6 hours away from me. I have put a deposit on the cottage and I was told that more would be required of me as we approach the date.

I feel like I'm being steam rolled. This woman is 10 years younger than me and I'm walking on eggshells around her. She is constantly throwing temper tantrums. She is very demanding and will say one thing but mean something totally different. I can't say anything to her without worrying about how she will react. The most recent issue involved my estranged mother. I was told through tears that my mother is going to ruin her wedding because she's wearing a white dress. I eventually bit the bullet and contacted her, even though we haven't talked in years. As it turns out, the dress isn't white and she had originally bought a different dress, but was told that she couldn't wear it because the brides mother didn't like it. And according to my mother, they're all the best of friends. I feel like they used me to be the bad guy, so they could remain friendly.

To date I have spent $1000 CAD on their wedding, and there's still a bill for the shower coming, bachelorette party, dress alterations (x2), tux rental hair/makeup, wedding gift, accomodations and travel. I want to do this for my brother, but I'm so distressed about the demands, rudeness and drama that it's effecting my day to day life.

I don't really have much family left. What do I do here? Should I back out, should I put up with it? I don't think I can even afford it anymore.

We are a one income household. My husband and I have worked very hard during our younger years so that one of us could be home with our children. My brother and his fiance both know this. My poor husband has been picking up mega over time shifts because we've had to spend so much money. I recently explained all this to my brother's soon to be mother in law and she basically brushed me off. Not once have I refused to pay for something, all I've asked for is a little more communication and respect.

I feel very sad and I hope someone can offer some advice.