He is close enough to be best man but not to have introduced his step children into his family.
What do you mean by he's close enough to be best man. It's his mum FGS. Of course her son is close enough to be her best man. Why wouldn't he be.
They're not his SC though. In the same way a partner is not a spouse.
Life happens...relationships end and people move on, but you can neither force or expect equal treatment from others in relation to stepchildren or partner's children.
You and your partner/spouse treating them equally is all you have control of.
This is a wedding, but where does it end? Like expecting equal value gifts to the one's she gives her GC?
If your DPs kids are getting excited about the wedding, you tell your DC it's the DCs GMs wedding. She's not your DCs GM.
When my DC were younger they asked why they weren't invited to an event their cousins were going to...as they're all very close. I told them it was a relative on their (cousins) dads side of the family, rather than their Mum's (my Dsis). They understood.
Would your DM invite his DC if she was getting married? Does she even know them?
A very useful approach I've adopted in life is to accept the things I cannot change and adjust my attitude, so those things don't have a negative impact on me. Otherwise you can end up extremely stressed out.
In the grand scheme of things, 2 extra kids is nothing, but I suspect the real issue, is she's not close to you...meaning your DC are even further away from her.
You have a DP who does a lot with your DC and you don't mention any issues with him, so making this a relationship issue as several pp have suggested would be extremely unwise.
your children have been deliberately excluded from a family occasion
The kids aren't her DPs mums family though. If this relationship ends, he could walk out of the door and never see the DC again. He has no rights to see them... as just mum's ex boyfriend and wouldn't have even if he was a stepdad.
To the pp who said DPs mum shouldn't be invited to their wedding (if they get married)... you need to get real. It makes no sense at all.
Why would his mum not be invited to his wedding, when he was invited to hers.
A sure way to kill your relationship is doing any of things in the last two posts. I'm sure they were not serious though.