You know, I remember someplace in the Old Testament (one of the early books) where God specifically says that the parents are not responsible for the sins of the child, nor are children responsible for the sins of the parents. I wish I was as good at memorizing Biblical references as a couple of Baptist friends of mine are!
When I hear about snippy remarks such as this by some "pious" people, Jesus' words come back to me---"Judge not, lest ye be judged...." We lay people are not called to pass judgment on who has what sin and what they should do. That is the job of a priest. And quite frankly, most, if not all of us, have got some time to serve in Purgatory after we pass on. I know I do, and I'd be willing to bet the rent that I'll see a lot of other people there! Stuff like gratuitous nasty remarks drives people away from the Church. It's not the spiritual work of mercy of admonishing a sinner, at this point in her life there isn't much that the poor dying woman can do about her adult children's falling away from the faith other than to pray for them, and maybe talk with them about her views on it.
It seems to me that we Catholics especially ought to be setting an example of Christ's healing grace if we are to sincerely claim Him as our Savior. A nun friend of mine sums it up well--in the RCIA lectures she gives, she states "Remember, you may be the only Catechism someone reads!" Recall that a Roman centurion commented--"See these Christians, how much they love one another!" Good examples attract converts. Judgmental, legalistic and condescending examples repel them.
Sure, there are lots of influences on young people nowadays and parents need to be aware of them, but the truth of the matter is that probably none of us weigh everything we say, do, and teach the next generation in light of what we are disseminating. Maybe your relative could have done better, maybe not. Maybe what she knows now, she didn't know when they were still in her house. Maybe she didn't know how to implement it, maybe there were serious obstacles in the way. Who knows? The Lord is willing to cut the repentant some slack, and only He knows the whole situation. She probably doesn't even know why things got so derailed. If the Lord is willing to forgive us, why aren't we willing to give each other the benefit of the doubt?
I'm not sure which of your relatives needs prayers more--the dying woman, or the cousin who came up with this comment. Never mind. That's a snotty remark from me. Pray for both of them, and the adult children concerned. They may yet revert, regardless of whether their mother has passed away or is still here.