Looking for advice on boundaries in a blended family

Hello!

Context:

My boyfriend (38M) has an 8-year-old son with his ex. We're together for over two years now. Over the past year or so, we've all gradually built a healthy co-parenting dynamic.

Things happened slowly and we were extra careful:

My partner and I had an early conversation about expectations if our relationship became serious. We agreed I'd take more of a "cool aunt" role rather than trying to replace anyone.

We waited until we'd been together for about a year before introducing me into their family dynamic.

I met his ex first, then met their son a few months later.

Since then, his son and I have formed our own relationship. Sometimes he calls me "Mom" or "Stepmom." When he calls me "Mom," I gently remind him that I'm not his mom.

We now have sleepovers when he's with us, and occasionally all four of us spend time together doing things like family game days.

His ex and I have also become friends.

Some additional context:

She doesn't have much family around, and we're Asian, where family support tends to be a big part of life.

She also doesn't have many close friends.

Because of that, I've found myself including her in my own support system—introducing her to my parents and friends, inviting her along sometimes, and generally trying to make sure she has people around her. I genuinely don't mind because I want a healthy environment for their son, and I also feel for her.

Recently, though, I had a heart-to-heart with my mom and her best friend, and they gave me a bit of a reality check. They said:

Her support system ultimately isn't my responsibility.

Even if things are good now, there's always a possibility that she could become uncomfortable with how close I am to her son.

They hope I still build a family of my own instead of centering my life around one that already existed.

They also reminded me that my partner's son will (and should) always come first.

I don't necessarily think they're wrong, but it did make me question whether I'm approaching this in the healthiest way.

So I wanted to ask:

For those who have a genuinely good relationship with their partner's ex or co-parent, how do you maintain healthy boundaries while still being close?

For those who've been in a similar situation, how do you stop yourself from overthinking the "what ifs" and enjoy the healthy dynamic you've built without constantly waiting for the other shoe to drop?