You aren't the asshole. It's your wedding and you are allowed to set your own parameters and boundaries. Since you told everyone months ago what your rules are, everyone has had plenty of advance notice to prepare.
It doesn't matter that Jen's original arrangements fell through. It doesn't suddenly shift the burden on to you to make an exception for her. In fact, doing that could make you the asshole to your other guests who followed your rules. That being said, if you really want your sister to be able to attend, maybe you could offer to help her in some way to get a new arrangement. I know you're already busy af with the lead-up to your wedding and likely stressed out. If you have the means, the entire problem maybe could be solved by offering to throw some money at it. But if that's not possible, you still are NOT the asshole!!
Ultimately it is Jen's responsibility to figure it out. It's your day! Also, it's only one day, and you can always get together with your sister and family for quality time, another time. If they can't accept your boundaries here, I wonder how they behave in other situations.