children’s mother doesn’t love them unconditionally, she continuously threatens to leave them if they engage in any positive way in dad’s relationship so they leave in constant fear of mother’s rejection…

This is absolutely, 100% emotional/psychological child abuse, and I would make sure the mother knows that. She should NOT be using her love as a bargaining chip against her kids. ever.

bf is a fantastic dad so I fully understand that he doesn’t want to cause the children any distress. The older child now starts behaving like mum, giving dad silent treatment at any mention of our relationship. I feel there’s nothing I can do other than wait for a change in my bf’s management of this situation but my worry is that there’s no clear timeline on the horizon….unless there’s a solution I don’t see? (Apart from leaving)

I agree that all you can do is wait or leave. Those are really your only options that you, yourself, can do. However, I think your boyfriend should put pressure on the mom to stop behaving this way because it’s not at all healthy for the kids… even just the idea of them thinking she’ll leave them if they don’t do what she says is abusive and unhealthy for them. So for his kids’ sake, if he hasn’t already, he should somehow attempt to get the mom to understand what this is doing to the kids… although I know sometimes that’s easier said than done and some people just don’t “get” it. But that might be where counseling/mediation comes in. It’s not fair for him to have to hide who he dates while she dates freely…. but if he were to just start bringing you around, he would become the “bad guy” to them, and if the mom is already practicing that type of emotional abuse, she may try to use you to alienate the kids from the father. So his only real options here are to push to change the mother’s behavior or just let it go….. which I do think will bite her in the butt later on, but at the expense of her kids, because this has to be messing them up emotionally, too.

One more piece of advice though… if he does end up talking to her, he should make it about the abusive nature of the comments she’s making to the kids and not about him being able to bring you around them.  If she stops saying those things and chills out for the sake of her kids’ emotional health, you will be able to come around as a result either way. If he makes it about you, though, she’s likely to not listen at all, I’m guessing.