I had a backstory for Angelina that I was using in a drabble once. It's based on nothing but my own thoughts, so no canon evidence whatsoever, but here goes.
Angelina's dad was a wizard and her mum a Muggle. He was a Quidditch player who was badly injured so became a coach. However, he wasn't a good coach and they had very little money. Angelina's mother never quite got to grips with Gamp's laws and couldn't understand why he couldn't just 'magic up some gold'. She leaves him (and Angelina) and marries a Muggle to become respectable.
I always thought of Angelina as an only child.
By the way George and Angelina do visit her mother with the children, but it's always a strained affair.
Pure speculation, but that's just the way she presented herself in my head.