Please note the unions of:
- Dorea Black and Charlus Potter
- Lucretia Black and Ignatius Prewett (Molly's maiden name)
- Cedrella Black and Septimus Weasley (#4 removed from the tree)
To add, here is a quote from Sirius Black:
The pure-blood families are all interrelated. If you're only going to let your sons and daughters marry pure-bloods your choice is very limited; there are hardly any of us left. Molly and I are cousins by marriage and Arthur's something like my second cousin once removed. But there's no point looking for them on here - if ever a family was a bunch of blood traitors it's the Weasleys.
Not only would this mean Molly and Arthur are distantly related by marriage, it would also mean Harry and Ginny are distantly related if Charlus Potter is indeed of the same Potter ancestry as Harry. And the Pottermore page on the Potter family tree isn't helpful. Is there any proof that he is/ isn't?