Where do I stand?

Jowo_2

I think the children, presumably aware that non-resident parents should contribute to their upkeep, are now of the mindset that you are morally obligated to cough up. I reckon they are demonstrating their resentment (either of your failure to support them or in reaction to the current strained relationship) through these financial demands, preying on your conscience, winding you up. Clearly they are hitting the spot because you are currently feeling punished.

You are an adult so should be easily able to put aside the malign influence of the adults involved. You should also be able to understand that your children are adolescents, possibly trying to please their resident parents by siding with them, too immature by age/experience to deal with a complex relationship. Don't conflate what their parents are doing with their knee jerk demands - keep it civil and ignore their parents.

McKneff

Perhaps you are getting the abuse from the step father because he is threatened, which is understandable, he probably feels that he is losing them to you so the only person he can take this out on is you. Its a perfectly normal reaction.
A Bit like an adoptive parent feels if their adpted child suddenly finds their birth parent and has contact with them.

And remember this = No one can make you feel guilty without your consent, so stop allowing them to make you feel guilty.

Good advice from Jowo there:T