Hi Magda,
When I first read your post, I was going to suggest disengaging, but then I thought about my own son.
When he was little, he would totally refuse to talk to adults, he was basically mute. Only this year, I realised that it's called "selective mutism". He's been living with my DP for 3.5 years now and they barely communicate. They say hello, if my DP asks something, my son answers "yes" or "no" and that's it. My DP has been bringing my son to school for 2 years now. Last year, he was bringing him to sport and giving him advice. But my son still doesn't feel comfortable enough talking to him beyond that. My DP obviously feels hurt and labeled him as "uncaring about adults", "uninterested", etc. But I know it's not true, my son does ask about him and even told me that he loved him, but he's extremely shy with him (yet, he's totally normal with me or his friends). This Father's day, he didn't wish him anything (the previous years, he would always bring him the craft they did at school, but this year, they didn't do it). My DP was hurt (not that his children wished me anything). The next day, it took me 15 min (!) of encouraging him to go to wish him something. He was too shy. After he did it, he said he felt good. When we eat together, there are 2 conversations: me and my Dp and me and my son. It's painful and dissapointing for me, I try to make them interact, it doesn't feel natural most of the time, but I continue...
I asked my DP not to give up on him. Because once he labels him as "uncaring about adults", it's easy to just give up. He even tried to stop saying hello "to see if my son will do it first", but my son won't. He'll just feel abandoned.
My DP is reserved and spends most of his time in his office in the basement. He always either works, does renovations or goes visiting his kids, so I do feel that there could have been more effort, especially since he's the adult. But I do work on my son and I know it's not parenting, it's his personality. I'm bringing him to a psychologist and things are better now (we noticed more elaborate answers to questions lol). Actually my son has a friend who's extremely shy, he's mute with me and if I ever ask something, no matter how easy, the answer is "I don't know".
My DP also tells me that his kids talk to me, 2 out of 3 of them are outgoing, but they talk about things they want and don't really listen when I talk and they have also been rude and bossy with me, which is not better. And I remember going to a chalet with them all + DP's mom and niece who came from Spain, and I felt absolutely alone. Nobody would engage with me, they would be either on their phones or talking between them in Spanish. So much for outgoing.
Anyway, I don't know what are the chances that the 3 of your DP's kids would be shy and I do hope that by 16/19, even a shy person can learn to have small talk, but I just wanted to offer a different perspective. I really do work on my son and the progress is very slow. If you suspect that it's the case there and not deliberate rejection, my advice is not to take it personally.
I also dreamt about a happy family, but even the "outgoing" SD hurt me with her rudeness and despite caring about her, coming EOW didn't make her feel like we are a family and I felt it very clearly when our baby was born. It's like there were 2 camps, daddy and his children (including the baby) and me and my son, as opposed to being a blended family.
My decision is to concentrate on my children, but not taking anything coming from SC personally and treating them well, because that's what a decent person would do.