Christ ate with sinners in order to preach to them and convert them away from their sinful ways.
Are you suggesting that the OP use the gathering as a preaching opportunity?
Otherwise, I see no reason for her to be required to attend.
I dont believe I suggested she was required to attend. However, if she is seriously interested in the boy she is dating, she will fast set herself up for intra family issues if she continues to avoid his relatives. It is always possible to have “another engagement”, but that plays out about so long and then it becomes transparent.
Christ ate with others; I don’t see much preaching to them. Teaching maybe; preaching? Don’t see it. Maybe to the Pharisees, most of whom he did not eat with.
We are all too concerned with telling others how they should behave. I call that preaching. Most people resent being preached to; most people who are open to trying to figure out why doing the same thing over and over is not giving them a different result are open to discussing it. Discussion of it is generally not a blunt “You are wrong and you are going to hell if you don’t change”.
I am all for helping others to turn from sin and live the Gospel. I have found that most of the time, indirection works much better than the direct approach. I can be extremely blunt if I think it will do some good, but most of the time bluntness just drives people further in the direction they are already going.
You want bluntness? OK, here goes. Maybe the OP needs to take a really long look at her boyfriend; if he doesn’t see anything wrong with it, maybe the whole issue will resolve itself by finding someone who is more attuned to her sense of morality (which is, for the sake here, correct). If she is uncomfortable with his relatives, what makes her think anything is going to change, or that he is not of the same mind? And why would she expect him to think like her on related issues? ABC, just to name one?
So, in short, no, I am not suggesting that she preach to them. I am suggesting that responding in a judgemental fashion (which is how this will be seen by them) will not accomplish helping them to move out of an objectively sinful situation. I am suggesting that she live faithfully to the Gospel; that is, if someone asks her why she is not living wuith her boyfriend, she be available to answer it; and that the answer needs to be one they can understand. Jason Evert’s sources are a good resource for crafting that answer.
But I do suggest she look at the whole of the situation; their living together is simply a part of the iceberg that is sticking up above the water.