delicate dove, have you already told your friend that she can organise your baby shower? if not, when and if she asks you can just say that your mum is organising it for you (even if you do most of the organisation she doesn't need to know). if you've already told her that she can organise it, then maybe you can be proactive in setting the date and venue, and if she says anything about "joining" in with the SIL, then just say that you'd prefer not to. you only want your close friends there, and not all of her SIL's friends as well...
on the photography front, i don't blame you! i wouldn't want anyone to be watching me either! having photos taken is already an awkard experience without being watched by a supermodel!!