https://archive.is/hJniv

So this post came up in relationships sub. In the older post, OP's sister confides to his mother and him that she had been cheating on her husband with her ex boyfriend and two kids who were born were not BIL's kids. Her mother supports her and tells her to never let the husband know. OP is conflicted.

In the second post, OP sees that telling the truth to BIL would result in -

1)BIL being made to pay child support in spite of the DNA test since he was already raising the children for the past 7 years.

2)BIL would lose his home and some money.

3)OP's relationship with his mother and sister would deteriorate.

He decides not to tell BIL in person.

In the comments, SocJusJessie says

Some countries have outlawed DNA tests without the mother's consent, and France outlaws them by any means other than court order. The stated goal is to keep society stable. I'll let you infer what that says about that society's women.

Lessons learnt

Don't get married. Don't have kids.

Edit - As pointed out by MagicGainbow, I should have worded the title better. The OP is initially reluctant to tell him in person but afterwards is asking for ways to tell him anonymously.