NTA

I think your engagement suddenly made your Fiancees sister look at her life and start thinking that 10 years is long enough to wait for her guy to propose, hence her ultimatum to him. (and if she is the older sister it could be she kept thinking she would be married first so this threw her for a loop).

I really dont think you need to worry.

First, her boyfriend is an adult and her saying she 'wont allow' him to attend if he hasnt proposed is ludicrous. He isnt going to leave you high and dry cause she 'wont allow' him. (and if he does, then he isnt worth keeping as a friend!)

Secondly, they have been together for a long time. Once they are engaged (if he does propose) I think it will be more of a 'ok phew! finally!' rather than so much excitement that your wedding is overshadowed.

And if she starts taking over your Bridal shower with talk about her engagement (or lack thereof) then just politely bring the topic of conversation back to your wedding. (and maybe ask your bridesmaids to help you do the same).

Dont let her engagement drama take over your thoughts. Her mother was rude in how she replied to you, but she is right in that the sisters drama doesnt have to be yours.

But I think its good that you communicated your concern because now they cant say they didnt know there was an issue.