True. But think…
Is the OP called to do anything about this? I haven’t seen anything that would suggest it is so.
Remember, there is a child involved. Is it right to keep the child from knowing his relatives? After all, he is completely innocent in this matter.
Also, we don’t know the whole story about either of these marriages. Maybe one, or both, were contracted without full consent? I don’t want to judge, and agree, it doesn’t look good. But, we just don’t know, and, most likely, neither does the OP.
We never fail when following the example of Jesus himself. He associated with corrupt tax collectors, prostitutes, and, I assume all sorts of sinners. The only people he was harsh with were those who were overly legalistic, preached false doctrine, and used the temple and it’s rules for financial gain. Those are the people he drove out of His father’s house. He never drove anyone who wanted to be with him, away from him. As he never sinned, himself, he was a wonderful example to people who were in the midst of sin. I can’t imagine anyone being able to sin in his presence, and, although not all became believers, I can’t imagine anyone who was not affected by his love and holiness. Even Judas tried to return the money he was paid for his betrayal.
I’m not qualified to preach a sermon, but I don’t think it’s right to make a decision about any of His creations, without following His example. Who knows? Bringing Jesus into the question. OP,…not by preaching, but by following His example, may help your brother and sister seek out Jesus’ examples, and hopefully, his will…for them to do the right thing!
Wishing the best for all of you, and God Bless!