Without drip feeding as this is a very specific set of circumstances I'll try and explain.
A male member of my family has been with a woman for about 4 years, always a bit 'off and on' as he won't fully commit by moving in legally (she would lose her benefits and he would then have to 'keep' the family) she has two teens from previous relationships. He does live there but not legally, he does pay his way food and bills wise but not as much as his GF would like and this causes tension which blows up into arguments every few months, hence the 'on / off' thing. He leaves, then realises that he's not really got anywhere to go so patches things up and she takes him back only for the cycle to rinse and repeat.
As a family the rest of us can see the situation but feel he's a grown man so needs to sort out what he wants for himself.
So, that's the background, the issue at the moment is a family event has been planned by a member of the males family - the male and his GF have been invited but her children (who aren't actually related to any of us) weren't thought of. This has caused the shit to hit the fan as the GF has taken great offence to this and made a fuss saying she felt her children weren't welcome in the family - in truth nobody really considers them part of the family and so they simply weren't thought about - none of us see them very often. The male in the family is now caught in the middle of the situation, in fact the GF has kicked him out until he stands up for her kids.
Is this common in step families? Do all the children from previous relationships need to be invited to events or is it usual that some families just keep to within their own family? Whilst I believe the problem is down to the fact the male has never really set his stall out in regards to the GF and kids, I'm finding it hard to believe the fallout from this and I'd like some perspective.