This whole scenario is not what I’d call “normal” and that’s not to say it’s bad or good, but truthfully speaking it’s just not common.
until the children are old enough to make their decisions
I feel this is potentially a bit open ended and subjective. When are they going to decide the kids are old enough to “make their own decisions” ? When the oldest is 12? When the youngest is 12? When the oldest is 16?
Even if at best you did get to meet them in the next year or so, would you be comfortable with this whole arrangement in terms of having the kids over regularly?
If you have kids before that his kids are “old enough to make a decision” how that will play out? Will you feel resentment if you want a lie in on a weekend and he’s off to see his ex’s kids and you need to get up with the baby? Will he prioritise your kids and if not, are you ok with that?
His ex is still in contact with his family and spends time with his mum sister in law and sister Iv been seeing him 6months.
Children aside, this is also not something I’d accept. She spends time with his family and you said he also spends time with her sometimes at the park.
It feels like he hasn’t fully moved on, even if he has moved on romantically he still feels like he needs her in his life in some way. I’m not sure if he’d be happy with it if it was the other way around.
Personally I’d definitely not want any part in this whole situation, and I’d just end things as I don’t think it’s my place to interfere with his current set up. But we are all different- as I said before while some people online may applaud him for this, you need to decide if YOU find this set up acceptable and if it’s going to work for you long term. And be very honest with yourself!