You feel these 4 offers were all pretty firm? If so, I'd contact each of them (maybe have your DH deal w/ your SIL) and say "Hey- I feel weird asking this, but in looking at my calendar, I need to get everything in front of me. You had offered to throw a shower. Now- if you no longer can, that's completely fine! I have no problem w/ that. But if you were still planning on doing it- I'd like to start looking at dates. My weekends are already starting to fill up through January".
If you want to, mention that you're now high risk too.
From this, hopefully you can get the ball rolling on getting dates set. And honestly, I'd probably try do to it through e-mail. Only in that if they actually need to bail on throwing a shower, it's easier on them to say no through email than over the phone/in person when you're saying "we need to pick a date". KWIM?