That plot hole didn't bother me as much as:

Credence being a Dumbledore. Really hoping that Grindelwald is just a liar because another Dumbledore doesn't fit into canon.

McGonagall not only being alive during this movie, but a teacher at Hogwarts.

If the Blood Pact was before "the duel" I guess Albus' role could have been defensive/healing or maybe he had forgotten about the pact or didn't realise its power so when he tried to strike at Grindelwald it rebounded onto his sister. Or maybe they kissed and made up after the duel?

Pretty sure G is lying to Credence, though did he say he was Albus' brother? Maybe a different branch of the family tree?

As for McG, could it have been the great grandmother (looking very well preserved) that she was named after? There is also a time travel theory: https://www.joe.ie/movies-tv/new-fan-theory-suggests-something-interesting-minerva-mcgonagall-new-fantastic-beasts-movie-649776