Robin doesn't look that old, she's probably like 42, so I imagine that she never actually had a first husband and instead got knocked up by someone while she was in high school. The high school guy predictably ran off and that's why he's not at all a presence in Sebastian's life. About 3 years later, she met Demetrius in college while she was studying carpentry and he was studying biology. He seemed like a kind and responsible man so she convinced him that they should get married young so that she wouldn't be alone raising Seb any more. Soon after they had Maru, but there was always a divide between Seb and Demetrius and the only people he got along with were Sam and Abigail even though they were about 5 years younger than him. So the reason Sebastian is so dour is because he never had a father figure and grew up knowing that he was a bastard while Maru had both of her parents.