How can I deal with and set boundaries with mom's siblings?

My mom is 2 years and four surgeries into her battle with colon cancer. She is just 55 and I am 29. I am an only child and committed to caring for my mom and helping her maintain her independence as long as possible.

My mom has 4 siblings. Her siblings make me extremely uncomfortable. I have had only cursory relationships with them since I moved away for college and, really, even growing up. They are all on the "unhealthy" spectrum (compulsive gambler, child molester, child molester, and anger management issues). At my mom's first surgery, two of my uncles came to "help". They inserted themselves into every conversation with the doctors, etc. They were very overbearing and my mother deferred to them a lot. Then, while she was in surgery, it was like they reached a breaking point and started taking their stress out on me in a huge, loud, public scene of yelling/belittling (precipitated because only two people could go back and see her after she got out of surgery and they wanted it to be them instead of me going first). Suddenly I was being screamed at for going away to college, being a bad child, visiting my father last christmas, etc., etc., etc. It was so crazy and really traumatizing. I have not had ANY contact with either of them in the two years' since.

Meanwhile, my mother has had 2 more surgeries and 2 rounds of chemo. She did her first round of chemo in NY and neither of these brothers could be bothered to help her to/from the treatments. (Though my cousins in NY have been VERY helpful to her). My mom does her surgeries at Cleveland Clinic which is about 2hrs from my home in MI. She stays with a family friend in Cleveland when recovering from surgery and doing this latest round of chemo. When she is out here I work Monday-Friday, then drive down on Friday night to give our family friend a respite, then drive home Sunday night to be back to work on Monday. Between her chemo treatments she comes up to our house, so she's been living with us every-other-week for the last 4 months.

My mother cared for both her parents (her siblings didn't help much), and I am committed to caring for her. I have a 2yr-old daughter and another daughter due 2/11/11. My mom's next surgery is scheduled for 2/28. The results are serious (if they don't get wide margins, etc.) but the risks of the surgery itself are standard.

My uncles are planning on "surprising her" by coming down for this surgery. This makes me uncomfortable, of course. Firstly, I don't want a repeat of what happened at her first surgery EVER. That shouldn't happen because I made my mom put down in writing who she wants to visit her, etc. However, I am also uncomfortable about the codependent relationships between the siblings, and feel that their boundaries are very strange (also worth mentioning is a history of incestuous abuse within their family). I don't have siblings, so I'm not sure what is "normal", but it seems very strange to me that they expect to be involved in her medical decisions (which I think should be private) and her conversations with her doctors (which I also think should be private). Instead of asking "how are you feeling?" they talk about what her white blood cell count was last. I also don't understand this need to be sitting in the waiting room during the surgery. I have to be there because I am her medical proxy, and will be staying with her in the hospital as her advocate while she recovers. But why the need to be here for the surgery, but not willing to be helpful when she returns home and I'm NOT there and she needs help?

Is it normal for siblings to expect this amount of influence over a younger sibling? is it normal for them to try to push the children aside in the process? I suppose I could understand if I were irresponsible or much younger, but I am a pretty well-adjusted, successful, career-minded, masters-educated adult.... compared to their collective smorgasbord of mental illnesses.

I'm sure the answer is in that last line (you can't judge the mentally ill by rational standards). I'm looking for advice on how should I cope with this, or someone who's had a similar experience? I can't seem to find any resources about dealing with your PARENT's siblings, it's all about dealing with your own siblings. I don't want to be exposed to their abusiveness myself (or my family) so I would prefer if they weren't around. But I don't want to isolate my mother from them, I just want them to have healthy boundaries. I feel they are a very negative influence on her in general. When they are not around, she is able to cope better and make her own decisions. When they are around, it's like she regresses to a little girl mental state. How/should/can I set boundaries for them?