If they're on the same bus, it's probably fine. Set a timer so they know when to leave, and make sure they are clear about closing up the house. In our neighborhood, the bus stop is on a corner of a side street, and there are a bunch of other kids with some parents out there. Th bus driver is very responsible and insists on the children standing in a line and not only waiting for the bus to stop and the safety bar to extend across the front, but also to wait for the "thumbs up" signal from him (meaning he has checked his mirrors to be sure no cars are trying to pull around him.
It might be a great opportunity to move the 7 year old into other areas of responsibility and maturity. And if there's maybe 1 more year before the 12 year old is on another bus, this is the year you build resilience in the 7 year old. That sets the stage for her to be able to help next year or even the year afterwards with the one who's now 5.
Everyone should have backpacks, shoes and lunch boxes by the door so the 12 year old isn't responsible for anything except saying, "Time to go. Grab your stuff."
Try going out to the bus with them the first day, then hanging back the 2nd day, then not going at all the 3rd day. If there's a problem, then you can always use day care.