I may get downvoted to oblivion but I did want to offer a counter point. Your BIL is father to these kids. Everyone says he shouldn't have to raise kids that aren't his, but, for all intents and purposes, they are. Blood doesn't make you a parent. Is your BIL really just going to waltz into the sunset and never talk to these kids again? He will still be responsible for paying child support at the very least. I have a very hard time believing he can and will just walk away like the children are nothing to him.
The kids are innocent. This IS their dad. I'm not saying your sister isn't an abominable human being. I'm not saying what she did was ok. What I am saying is that these kids will be destroyed. Are YOU going to be the one to look them in the face and say their daddy isn't their daddy? Why would you put that on your BIL, if you yourself wouldn't do it.
Your sister should have told her husband. But she didn't. And frankly I don't see how this is any of your business. This is between your sister and her husband.
As someone who has been cheated on, it is not your place, at this time, to tell. The time to tell would have been in the beggining, not 7 years later. If I was in this husband's shoes, I personally would never want to know.
Like it or not, this is his family. Reddit hates cheaters so much, many here often fail to look at the human element because all people here care about is watching cheaters suffer.
No one here is thinking of innocent children that will be completely broken and confused. They are far too young to wrap their minds around that daddy isn't daddy. Because to them, he IS daddy.
Also, imagine if you tip off anon. Let's say your BIL confronts your sister. Do you REALLY think she won't know it was you? Trust me, this will come back.
If your BIL wants to do the test himself from his own conclusions someday, that's his business. If he doesn't, again, that's his business.
So, he finds out, lives are forever destroyed, and he still has to pay. Or, you know, he may decide to stay with your sister. It can and does happen. You know who they will cut off? You.
This knowledge does not affect your life in any way. It is not your business or place to tip your BIL off. And those kids ARE still your niece and nephew, regardless of who the sperm donor is. The only outcome for them is being emotionally destroyed and possibly abandoned by their dad. Everyone WILL know it was you who told.
Let it go.