You are confusing Donatism with receiving communion in a state of mortal sin. The sacraments are not invalidated because the priest is a sinner.
Let’s review the Bishops words.
“These people are objectively living in a state of mortal sin and may not receive Holy Communion. They are in great spiritual danger. At the best — and this is, sadly, often the case — they are ignorant of God’s plan for man and woman. At the worst, they are contemptuous of God’s commandments and His sacraments,” Sheehan said in the letter.
If a priest is doing the same thing by screwing his cousin for decades, surely his is living in Mortal Sin as well.
What does the Church say about priest saying Mass who are in Mortal Sin?
“To celebrate Mass or receive Communion while in a state of mortal sin would be to commit a sacrilege.”
Canon 916 of the Code of Canon Law states: “A person who is conscious of grave sin is not to celebrate Mass or receive the body of the Lord without previous sacramental confession unless there is a grave reason and there is no opportunity to confess; in this case the person is to remember the obligation to make an act of perfect contrition which includes the resolution of confessing as soon as possible (see also Canon 1335).”