I think my mum's marrying the wrong man

How painful this must be for you. You’re trying to be supportive by pointing out that this man seems to treat her badly and she won’t listen. I agree that the behaviours you describe don't bode well for the future and I can completely understand why you and your sister are feeling worried. However, I do need to raise something which at first glance may seem a bit challenging.

I’m sure you’ll know that sometimes it’s really difficult for children of any age to see a parent re-marry. On occasions these feelings can be so overwhelming that the meekest of potential step parents can appear monstrous. Obviously I don’t know the specific circumstances for your family, but it’s always worth reflecting on our own motivations for disliking other people. That said, if you remain so concerned, I think you’re right to raise this with your mum - but it may be helpful to think about the best way of doing this.

The bottom line is that by being critical of her fiance, you’re also implying she can’t make decent choices. Unsurprisingly, that can make anyone feel defensive and attacked. I'd invite you to think about how she might experience your concerns, particularly if she sees the situation very differently.

Often, once backed into a corner by disapproval, no matter how well intended, any of us might find it too difficult to tell a loved one what’s really going on. That may account for the defensive responses you say you get from her. It’s possible too, that she may fear she will ultimately have to make a choice between him and you and your sister and of course, this may be a fear that you share with her.

It sounds like you and your mum could get caught up in a one of those conversations that just spirals down until nobody feels heard. So you need to change direction. I wonder if you have focused on the negatives you see rather than asking how she actually feels about him? If she can start to see you as genuinely interested in her side of things, she may feel more able to be honest and that may achieve several things.

First, you’ll know if she actually wants to stay with this man. If not, you can help her to get out. Second, she’ll know you care about her feelings. Third, she should find it easier to hear what you’re worried about and fourth, you’re helping to keep the conversation open in case things get worse.

It’s very difficult to see those we love treated badly but you can’t control your mum and what she wants to do. Ultimately though, if this turns out to be the beginning of an abusive marriage, knowing she has your love and positive regard may be the thing that most helps her to leave it.