When my brother was in this situation, I felt I had to say something. For one thing, his son and “step-son” were my Godchildren. So, I did–I think I did it very gently and with great love. He ripped my head off. It was horrible.
There’s a Bible verse that says that if you don’t rebuke the sinner, you will be held liable. I’ll see if I can find it.
Eczekial 3:1819 If I say to the wicked, you shall surely die, and you give him no warning, nor speak to warn the wicked from his wicked way, in order to save his life, that wicked man shall die in his iniquity; but his blood I will required at your hand. But if you warn the wicked and he does not turn from his wickedness or from his wicked way, he shall die in his iniquity; but you will have saved your life.
rebuking the sinner is a work of mercy. But, maybe you should ask your confessor. They are usually very wise about giving good advice in families.
It did bother me very much to see my brother shacking up, making babies, and the rest of us pretending it was all normal and he wasn’t losing his soul.