met my BF son and he is now attached to me

I'm in a LDR with my boyfriend who happens to be a widower and is an amazing father to a 10 year old boy who I absolutely adore. We have been dating for a year now (he's been widowed for almost 2 years) and I finally met his son 3 weeks ago for the first time. His son and I hit it off. To the point where he'd run into the living room after he got off the school bus and show me everything he did in school. The 2 day I was there my BF was putting him to sleep and I stayed in the kitchen when his son came back down and said to me can you come put me to bed with daddy. I looked at my BF and said ok I'll come. And both my BF and I put him to bed together for the week I was visiting. When I came back to the east coast, they live in the MIdwest, his son Facetimed me and said I was the most exciting thing in his week. 2 days later I got some drawings he made with "we miss you" all over it. The 3 of us, his son my BF and I have a routine now for the past 3 weeks. He calls me after school and before bedtime. He recently asked me when I was coming back to visit. He said he didn't miss me bc we talk everyday but at the same time he does. My BF wants our 3-4 month visits to be a once a month visit. 
I feel like our relationship has now changed. Before it was me and my BF. Now it's the 3 of us. Last night his son flat out bluntly told me you and daddy have this parenting thing down pact. You are doing a good job. I didn't know how to take that bc I don't have kids of my own. My BF says I'm doing an awesome job with his son and that he loves seeing his boy laugh again. I love my BF and I love his son. Another big thing is he has a stepdaughter from his LW who still relies on him for everything. I have not met her and know had is going to be a problem when we do meet, giving the factor her stepdad moved on.  I  just worried she's going to ruin my relationship with his son or make all of us feel uncomfortable bc his son is now attached to me, daddy's gf. And say I have no business trying to raise a child giving I don't have any and his son isn't mine. Am I just overthinking and over analyzing it all??