Veritas - I am also one of those people who do NOT want a 3-ring circus at my births. I'm very much in need of peace and quiet and the freedom for it to just be me and my husband. My first birth was midwife attended - and I LOVED both of my lovely, unobtrusive midwives... so that worked out very well. One was a bit too chatty though (the assisting midwife who I hadn't really spent much time with before) - she realized I was Christian about 1/2 way through from my book collection on the shelf, and started talking faith with me and I just about wanted to throttle her... it was just not the time, and seriously, at that point, I was in really active labor. I kept my answers brief, but polite, and she eventually quieted down and took my social cues pretty well, but I still have memories of how challenging it was and how much energy it took to handle that conversation. I bet, for some women, conversation can be distracting - but not for me.
I loved my second birth, which was just me and my husband. It was really peaceful and I was totally uninhibited - and it was magical to meet our daughter, just the three of us in the middle of the night. We had a friend on deck to call if we needed help with childcare or anything else, but ultimately, we didn't call her until after the birth to come over and help with clean up, cooking, etc. I chose her very carefully too - I knew I could be around her naked, totally out there, and she is such a totally peaceful and accepting and lovely person... and who is totally outside of herself and cued in to other's needs. I'd ask her again but she lives in Germany now, dangit!
Anyway, all that to say - I would not hesitate to follow your instincts and gently, lovingly, insist on what you and B want for the birth of your daughter. If that means your sister is not the doula, and you ultimately decide you just want it to be you and B - that's ok. I don't know that I'd want a 5 month old at my birth either.