If you usually pay the bill and want to change the status quo for whatever reason you should have a discussion well before the lunch date with your sisters informing them of your intentions. That would not make you an asshole.

If you wait for the bill to come and then inform them they will be paying their share, all the while knowing they don’t have the money to pay it would make you the asshole.

In my opinion the lunch dates and the loans are two separate issues and should be treated as such. One has nothing to do with the other.