[[WMG: The Charming Family Tree]]
This is an attempt to unravel the web of extremely complex and convoluted character relationships in this, especially with regards to Henry's family. There be much Fridge stuff here.
Henry is the son of Emma and Baelfire. His maternal grandparents are thus Snow White and Prince Charming, and his paternal grandparents are Milah and Rumple.
Regina is the adoptive mother of Henry. She is also the stepmother of Snow, making her also Henry's step-great grandmother on Emma's side, and Emma's step-grandmother by extension. This would also mean that Cora, Regina's mother, is their step-great great grandmother and step-great grandmother respectively.
No wonder the characters on the show spent all of five seconds trying to work this out. It'd drive anyone mad.