I would definitely say something. This is YOUR wedding and you don't need the extra stress. Try and say it in a nice way, maybe something like "I know finances might be a little tight for you with having another baby, and I know things can be stressful with having/trying to have a baby, so I want you to just focus on that instead of being a bridesmaid." Kind of make it sound like you are looking out for her instead of "firing" her as a BM. I went through the same feelings...had a total flaky and unreliable BM and she stresses me out MORE than anything else.
We postponed our wedding last spring, so when we made the decision to go forward with a DW, I made sure to emphasize I was keeping it SIMPLE and only having my sisters this time around.