M.F.
I would say no. It's not ok to be selfish this time, or any other time, when you're talking about a child with a terminal illness.
You're pregnant, not sick. You're uncomfortable, he's in real pain. You're tired, he's on medication that is literally poisoning his body in hopes of killing the cancer cells, while the healthy cells die too.
You have other days to relax and veg on the couch. Go to the early Easter, go to the benefit, leave a little early if you need to, but don't skip it to revel in some alone time and napping.