Mothers and daughters (and parenthood in general) are a major, driving theme inOnce Upon a Time, from Emma's initial journey to Storybrook through Regina's descent into evil and Snow's reunion with her grown up daughter... and while these relationships are moving and compelling, not a single one could be called "conventional."
Considering that the show is based on tales where motherhood is either entirely traditional, twisted and corrupt or completely absent, this is admirable.
The most prominent mother figures on Once Upon a Time are protagonist Emma and "evil Queen" Regina, both of whom feel love and affection for their son Henry. Their son Henry, because (at least, until his relationship to Charming was revealed) Henry has no father figure in his life. Just two mothers with radically different backgrounds and perspectives who both want the best for him. One is his adoptive mother, the other his birth mother who felt compelled to give him up for adoption so he could have a better life. Both love him and are wiling to fight for him and protect him. And although Regina isn't exactly the best mother figure, as an evil fairy-tale queen, she's never treated as "lesser" because she is not his mother by blood, and once it got over Emma's initial lie-detection test, the show does not suggest that she loves him any less. Meanwhile, the show never attempts to criticize or shame Emma for her own decision or suggest that she is a bad mother. Neither is a perfect mother figure for Henry at the start of the show... but that is part of the compelling nature of the story. Motherhood is certainly a part of their story and one of the factors in their decisions, but it is a part only. They have lives and needs and flaws outside of it.
Then there's Snow White and Emma... a woman who was forced to give up her child, and a child who grew up believing she had been abandoned. Oh, and they're now the same age as each other, and became close friends and roommates before they realized they were related. How much more unconventional can you get? And although they love each other, they didn't immediately adopt a mother-daughter relationship when they found out the truth. Emma still has scars from the past, and they have a lot to talk about... they protect each other, and care for each other, but that is a relationship that is never going to look "normal." And that, in the context of the show, is OK too. Because no parental relationship is normal. Ruby lives and works with her grandmother. Charming grew up with only his mother, who he lost so that he could have a chance for children of his own, and is now acting as grandfather to a child when, from his perspective, he only had a baby daughter a few weeks ago. Belle only has her father, who doesn't trust her to make her own decisions, and even Aurora, who has no family left, develops a kind of little sister bond with Snow over their shared nightmares.
And that's not even getting into the mess between Regina and Cora.
Motherhood is really important in fairy tales, mostly because of its absence. "Real" mothers (aka birth mothers) are usually dead or unmentioned, and many female characters get their happily ever after by marrying and becoming "real" mothers themselves. Other "mothers," especially step-mothers or simply older female characters, are witches, jealous of the younger girl's beauty, desperate to punish them and ultimately irredeemable villains. Yet in Once Upon a Time, the only purely "traditional" mother we see is Cora, the irredeemable villain figure desperate for power and advancement. Everyone else has something unconventional about them... and although they are not perfect angelic mother figures, they are caring and compelling characters. In many ways, the most unconventional aspect of motherhood in Once Upon a Time is that it turns one of the most pernicious -- yet subtle -- sexist fairy tale tropes on its head, allowing women to be both mothers and people, without criticizing them for their independence.