Hello all... brand new here and well, I am looking for some advice.

Some history: I currently have a 2 year old BS. My son's father and current SO, has two other children - another 2 year old (SD1) and a 1 year old (SD2), both boys. My son is the oldest.

To make a long story somewhat short - SO and I have had an on-and-off again relationship for the past 5 years. Our longest off period, in which I became pregnant, was originally permanent, I moved to another state for work after college and we decided that we did not want to pursue a long distance relationship. We stayed in very limited contact. He did visit 3 times, but nothing serious. Of course, on his last visit, walla! I end up pregnant (considering I had been sleeping with this man for 3 years at this point and on the same birth control with NO scares, it irks me I fell into the 2% failure rate at this point). Oh well, the only 100% birth control is abstinence. I had known he was involved with another woman sexually because we have always been open and honest about our dealings just due to the nature of our relationship. We waffled back and forth about keeping the baby (almost 3 months), before deciding we'd like to be parents and that we'd discuss in the coming months about giving some serious consideration to having a stable, monogamous relationship. Wouldn't you know it, a week later the other woman finds out she is pregnant.

I spent all of my pregnancy and the first year our son's life two states away due to my job (dream career and I was/still am the breadwinner). The woman who ended up pregnant shortly after me is batshit motherd*cking crazy... suicide attempts, withholding their child because he doesn't want to be in a relationship with her, threatening court, threatening to call the police for kidnapping when he has the child and he goes out, moving, claiming he hit her and filing for DV charges, etc.. etc... January of this year, SO and I were talking about me moving closer, everything worked out fine, I found a job in my career field, got an apartment, etc and the crazy just ramped up with this female.

Consistently putting my foot down and creating boundaries and refusing to interact with BSC BM and limiting SD1 over my house has really made things minimal. SO has also called her bluff on moving, court, etc which has taken a lot of wind out of her sails. She also knows I will in no way, shape or form, roll around in the mud with her via phone games or text messages.

My relationship with BM2 is friendly and amicably. We hang out from time to time and get the boys together. SD2, I enjoy immensely and I could really see myself loving him as my own. He also looks a lot like my BS and they have similar personalities.

Due to BSC BM and her terrible, awful parenting, I do not care for SD1. I have a HUGE feeling that SD1 is not SO's, he looks NOTHING like his brothers or SO, but that's not my place and if SO claims him, then I can't do anything about that. Additionally, most of my dislike for the child is due to his mother. BSC BM has threatened violence and name calling SD2 as an infant (she called SD2 a piece of sh*t) and has terrorized BM2. Either way, SD1, he's not a bad child, but VERY undisciplined at his mother's hands who constantly undermines SO, and I know he's a toddler, but as an example, we took all 3 boys to visit FMIL before she flew back to her home island. SD2 was just starting to walk so he stayed close to the porch. BS and SD1 were playing, not far, from the porch. They were getting close to the street and I called for them to come back. BS immediately turns around and comes back to a comfortable distance. SD1 just keeps going like he didn't even hear me. SO calls him and starts to walk towards him, SD1 looks back, grins and takes off at FULL SPEED for the road with a car careening around the corner at the same time.

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With that back history, what advice can you give me for being a future stepmom?

SO and I are serious about moving forward together as a couple and SO understands that he is the parent. He does all of the work for his children, but I feel as his future life partner, we're in this together and as long as respect is the forefront, I have no problem playing an active role in the lives of his other boys.

My SO has been proving that he will have my back when it comes to the kids. He disciplines SD1 on the rare occasions that he is in my home/presence and reinforces the expectations I have for BS onto SD1 because earlier on, BS picked up some of SD1's nasty non-listening habits and due to the fact that our child rearing mindsets are similar.

I am NOT rushing into this relationship as I hate drama and there's still a bit that needs to be done before I will say yes to him 100% such as squaring away a custody order with SD1 and BSC BM so the nonsense ceases. Despite being with SO, albeit unstably, for 5 years, we are laying a true foundation for a solid situation and I've requested that we go to counseling.