Op, so sorry to read that your mum's ill. It must be a really stressful time for you. 
It sounds to me like it might help you to be able to take some action now in order to ease your stress.
One thing you could do is to contact your bil (maybe get your fiance to speak to his brother directly) to say that you'd really like them to come to your wedding but hadn't expected them to be back in time. You're still in the process of juggling the numbers but would love it if they could come to (as applicable) the ceremony and the evening do and that you will let them know if any spaces open up for the meal.
As for your mum's friend... could it be that she is thinking that whilst she wants to support your mum, she wouldn't be comfortable going if your mum isn't there? I can't imagine how I would say this if I was her. Maybe you could contact her with something like:
"Hi 'friend'!
Hope you're well. Sorry to be contacting you again so soon but I've realised that I misjudged the RSVP date and we really need more time to plan things, especially with Christmas so close. So sorry to mess you around but do you think you could let me know by Friday if you're coming? Many thanks, OP xx
RSVP Friday, 23rd November"
Alternatively, you could try to chill out and let it all unfold?? (I know it's difficult though.) I think if it was me I'd speak to the brother and make sure he knows how wanted he is, that you're at capacity budget-wise but that you're going to do your best to fit him in if he can make it. Take it from there.