You can find the complaint (which may or may not have already been filed) at this link.
Some of the plaintiffs are members of Congress who are not parties to the other litigation, so the Rooker-Feldman doctrine may not apply to bar their suit. But there are other abstention and standing doctrines that will stand in the way. Basically, it is hard to get a federal court to act to essentially enjoin the actions of a state court when there is a direct route to go after the state court action to the Supreme Court (which some PA legislators have already done (or apparently done)).
So it is not clear a court will reach the merits. On the merits, the complaint raises only Elections Clause issues, which appear to be a long shot for reasons I’ve explained.
The complaint requests a three-judge court. That’s important because it provides a direct route to appeal the case to the Supreme Court for relief.