I know it's the 21st century and it's his baby too, but why would father-to-be be engaged in planning a shower for his own child? Sounds a little self serving...that aside...or not, can you contact your sister's best friend, chum from work, (somebody, anybody!) and shoo BIL out of the picture for the planning? You could use the premise that he *shouldn't have to* organize his own baby shower and Best Bud and you have things covered. If you had a really good friend, could you even say "I owe you one. I'm duty bound to this task and I need your support."
As for detachment, I would not bring it up unless asked. I had someone detach from me a couple years ago, made me sad, but I saw and understood what was happening. I thought about asking, but searched myself and realized the relationship as it had been had run it's course. There were differences that won't be resolved. Telling me why would just have been very awkward and made me sadder. She's not a petty person, we're on different paths. We're still Facebook friends, and run into one another now and then, and treat one another politely. There's no benefit to her *explaining why* I'm being rejected from her circle of friends, is there?