Step children dramas - Should I stay or Should I go

Ok, Il try to keep this as brief as poss. My fiancé and I have been in a relationship for 15 years, which started from telephone conversations on a chat line then grew from then onwards. It all started off with a few lies here and there where he told me he was 23 (I was 18), he was actually 25 which he told me a few months later. We spoke on the phone for approx. 1 year then we thought we would meet up, anyway to cut a long story short, he has 3 children (which he didn't come clean to even after I moved in with him, he told me that they were his sisters children and he got custody of them). 15years down the track we have had some great times as well as some not so great times, his 3 childrens mother hasn't been in the scene for close on 12 years and all of a sudden, the 16 year old decided that she wanted to see her mum (this was really heart breaking for me as I have been their mother figure for the past 12 years when their mother decided that she didn't want to keep in contact with them anymore), the 16 year old packed a mental and wrote really really horrible things on her facebook page which were targeted at her father and I, but after some stern and firm words with her and complete shutdown of all of her luxuries and tight pressure on with her schooling, she has since become more positive about life in general. Now that we have her under control her older brother (20) has decided to go behind our backs and not only contact but make a surprise visit to see his mother and invite her to a Mothers day service at his church. The disappointing thing with this, is that he couldn't even talk to his father about what he was planning on doing, he feels that he doesn't have to tell his father anything... All very true, but I feel that he should have respected his father a bit more and mentioned what he was doing (I found out about this through, ok a bit of snooping, I am sure any other good mother will do this just to be sure their children are being safe online). I told my partner about the visit so he gave his son the opportunity to reveal what he had done. I know for a fact that my step son has told his father about this, but my partner has told me he hasn't... why would he lie to me about this? I have always had issues with the 2 older boys in terms of them not accepting me, they don't like to accept me as their mother (fair enough I wont force this on them), I have spoken to my partner about this numerous times but he just thinks I should try a bit harder. Its got to the stage where the oldest boy doesn't even talk to me anymore, the second oldest is getting to that stage where whenever I am in the kitchen they will walk straight out. I feel really hurt about how and what they have become as I feel responsible. My partner and I have a 7year old and now I feel that I have to stay in this relationship, so that my son won't grow up without a father.... HELP ME, WHAT DO I DO? I feel there are more negatives than positives to this relationship, I love my partner soo much I can't bear to leave him.....

Hi SUSIEDQ, thanks for your response. Just to answer your questions: there was always visiting rights for their mother to come and see the children, my partner and I never took that away from her; when the kids were younger they would always be very difficult and sometimes upset after visiting her, but we comforted them and told them that they will see her again soon, but once too often their mother wouldn't show up, wouldn't ring them until eventually didn't make further contact with them for 12 years. During this time they became very distant children. Then out of the blue the mother makes contact with them on social media, and this is when they started to go behind our backs. So everytime they have made contact with her their moods change, their attitudes change, they become distant again all the very similar to how they were after visiting their mother when they were younger, this is where I want to protect them from going through the same disappointment and pain all over again. But as you say do we just let them? Hopefully the mother has changed her ways and will actually be there for them. Her children were always second priority to her when she was with my partner, she preferred to be out drinking and gambling and having a night life rather than staying home and looking after and taking care of her family. She also has another child who she also has no contact with and then another 2 that she has with her in a current relationship. I hope and pray that she has changed and doesn't let the kids down again.