I didn't make it to the end of your post because it was so long and not broken up into paragraphs so very difficult to read. What I got was this - your SO moved to the US after marrying his ex and having a child, there is a lot of he said she said baggage between them, there are behavioural problems with the children including stealing, which SO has not dealt with very effectively. You have been together 2 years.

2 years is early days in the life of a stepfamily, if SO can't address rule breaking and behavioural problems you will find it increasingly stressful and difficult to be involved with his children. I would carefully assess whether your SO is willing and able to enforce some boundaries with them before you consider taking your relationship to another level and tying yourself into this family. If you don't trust people who spend time in your home and have to hide your belongings that can really eat away at your peace of mind and cause a lot of resentment, which is destructive no matter how much you love SO. In a stepfamily love is not enough - the nuts and bolts of trust, support and discipline need to be in place.

You cannot control BM, and there are always 2 sides to any story, your SO may not be whiter and white in his actions towards her. He is responsible for the relationship with her and for your own peace of mind you should try not to get involved. It doesn't help anyone to trawl over 10 years worth of events, you have to try to deal with the situation as it exists now.

Repost with some paragraph breaks so we can read this. I can't get through the mass of unbroken text.

So far, it sounds as if you are in a challenging situation.

Good luck.

I didn't make it even a 1/3 of the way through, and all I came away with is her SO likes to spin some tales about his ex-love life. Who cares about all the back and forth?