Applying the model with family life situations

My family members don’t know about the model and when we have family discussions, I find it hard to explain that we are responsible for our emotions.
They say it makes no sense and shut me down.
For example: my daughters (19 and 14) aren’t invited to a cousin’s 19th birthday party and my husband and especially my 19yo are finding it odd and unfair. Because she would invite her cousin to hers. Both families are staying at the holiday homes near by.
I said it’s his personal choice to invite whoever he wants and the best way is to accept that it might be that he doesn’t want her (them) there. It’s not the first time he hasn’t invited her to his birthday or party.
They think it’s wrong. Because it’s a family and cousins SHOULD (I don’t like this word) be included.
My husband wanted to send an email or a text about it to him.
How do I take part in this conversation and be helpful with the Model without a coaching approach?