Your daughter would have been wise to answer “yes, we got them”, and left it at that, but she didn’t. Oh, well. NFP is a means by which a couple with a serious need can attempt to avoid pregnancy, but it is not a requirement for all married couples. Think about it: for how many centuries of Christianity did this method not even exist?
They may not have any reason to avoid pregnancy, her husband may be sterile and they don’t care to tell the world about it, etc. etc… To be blunt, they may also think that NFP is too often used with a contraceptive mentality. Who knows? They may be scandalized by your assumption that they ought to use it when they don’t feel they have any serious reason to avoid pregnancy! There are some very serious Catholics who have that concern about NFP. At any rate, if you were to jump to the conclusion that they are conducting their sex life in an immoral way, you would be guilty of rash judgement.
You have done your duty by seeing to it that your daughter and SIL have been educated in the faith and have been given the information needed to avoid pregnancy* if they ever have a serious need to do so*. If your counsel is sought, of course you should answer according to what is moral, but if your counsel is not sought, you do have a duty to give your daughter and SIL authority in their own home.
You also have a moral obligation to avoid rash judgement and to avoid making statements to anyone that knows them that could lead others to rash judgement concerning their situation. Even if you have concrete knowledge of the sins of others, you may not speak in a way that would harm their reputations unless there is a compelling reason for you to do so. You don’t have that reason here.
Pray that their marriage continually increases in love and holiness, and pray that you may leave behind anxiety about their unknown choices in favor of trust in God. That is the best thing for you to do.
