Take yourself out of this situation and look at it from the outside. It's actually pretty simple and you have a few options.
A. Stop letting your mum control everything - it's your wedding and not hers. Either get cheaper dresses so you can afford to include everyone, or spend your own money, or your fiances, or the mother of your fiance or whoever else's money is on offer, to get more dresses.
B. It sounds like your future sister-in-law is excited at the prospect of being a bridesmaid. Worst case scenario, ask her to pay for her own dress? I'm sure she would rather pay for her own dress and actually be in the wedding, than not.
C. Mums can be controlling. Let her know that it isn't her decision. I get the feeling this isn't the only aspect of your day that she wants done in a specific way, and it's not fair. Your day, your memories, your photos, your celebration, your wedding. Your mum is there to support, not control.
A. Stop letting your mum control everything - it's your wedding and not hers. Either get cheaper dresses so you can afford to include everyone, or spend your own money, or your fiances, or the mother of your fiance or whoever else's money is on offer, to get more dresses.
B. It sounds like your future sister-in-law is excited at the prospect of being a bridesmaid. Worst case scenario, ask her to pay for her own dress? I'm sure she would rather pay for her own dress and actually be in the wedding, than not.
C. Mums can be controlling. Let her know that it isn't her decision. I get the feeling this isn't the only aspect of your day that she wants done in a specific way, and it's not fair. Your day, your memories, your photos, your celebration, your wedding. Your mum is there to support, not control.