Eliza that really resonates with me.
I actually met an OH with uni age kids (you can't even argue that I was disrupting their childhoods)
long divorced - mum left dad.
but their mum wasn't happy with her new relationship - she was very attractive in her youth, met a younger man (good sportsman but never made any money out of it), and thought of saw herself as some glamourous type who got a toyboy who was drawn to her "beauty".
but along the line she ended up working lunatic hours to maintain him and their son together and resents it.
The daughters decided to take their mothers bad marriage out on me - I am (and was when dating their dad) a content bluestocking childfree type who enjoys all the social flexibility that brings.
I made choices so I can live like a hippy and be creative and work part time.
I think they hugely resented this about me( to the extent of suggesting I raise my working hours, didn't have "date nights" with dad or do anything nice
).
transferring the lack of attention and the unhappiness of their mother onto me as an easy target?
It was like I was expected to be the "emotional fall guy" for all the family shit and insecurities. The daughters would make comments about my "doc marten lesbian shoes" (tasteless enough in itself) and I'd be caricatured as this Plain Jane jealous weirdo to make their mum look good 
And although he had some good qualities which is what drew us together, I think dad quite liked this dynamic - he liked deluding himself he had this ex-wife and daughters who everyone was "jealous" of.