I’m a he not a she…

This is my point. The reason I was asking was I was confused as to being a Catholic are we not obligated to point out to someone they are in a state of mortal sin and should the priest who is marrying them say ok quit living together for a couple of months go to confession and I’ll marry you. Please no replies saying I’m judgmental, I’m not judging them. I’m sorry its not right to condone this and let it continue right up to the wedding. This is why I’m asking if it is permitted to attend
Oops sorry about confusing you for a she
The priest ideally should, not just for the sake of the couple but in order to correct the confusion that the couple has caused to fellow Catholics, ask them to separate.
In this case, the real reason it has come down to you having to choose or worry is because the priest had not done that. If the priest had done that, the couple had not followed through (which could imply they do not understand what a marriage is and Church teaching on sexuality). Or the couple did follow through and are living as brother and sister after a good confession. But in that case, the damage (by adding to the normalizing of cohabitation before marriage) they had done publicly during their years of cohabitation have not been repaired. So the confusion still remains among those who knew them. Ideally they should have separated so that there is a visible acknowledgement that they did something wrong and they do not consider it an ok thing to do.
Since these things have not happened, you are left with having to decide whether you want to attend the wedding and how much you attending will give the impression that “cohabitation before marriage is normal”.
If I were in your shoes, I would try to do what my conscience and prudence informs me.
You might want to ask yourself what impact you will have in not attending. Will they understand why you will not attend. Is it possible that they will think of it as you not agreeing to them marrying (which you are not doing. you only disagree with their cohabitation).
On the other hand, if you have young children, you might want to worry about the message you send your children. They might one day think it is ok to cohabit because you will just be ok with it as long as they get married in the end. You not attending will at least send them the message that if they want your approval, they should not do such things.
The truth is, unless there are those who look up to you like children/grand children or younger siblings, or you are someone people look up to in general, you attending or not attending will most likely not make an impact. It was always the priests job to correct the damage and make it clear to the couple. If he failed, then at this point, you cannot do much. But if you do have those who look up to you or highly value your approval, then I feel it will be better not to attend. Not for the sake of the couple but in order to avoid scandalizing those who look up to you.
What you could do now is try and talk with them even now to perhaps separate temporarily because cohabiting is not moral (this depends on how much you know them and how friendly you are with them). If you hear from them that the priest said its not needed, then you can take the complaint to the right man and make an impact.