I don't think it is that odd.
Emma is a mass of contradictions. She can be manipulative, calculating, nasty, dangerous and violent. But she is also capable of kindness, compassion, empathy and guilt.
I'm not sure she'd wish a dead child on even Moira and does see befriending her and 'protecting' her from.Cain as atonement for James death, partly because it is what James would have wanted and will be hard for Emma given the history.
History is also the reason Moira is accepring ut. She probabky feels she owes Emma so if being at the farm and talking to Moira helps her through James death, she'll let her as she is basically a kind woman.
It is also probably nice to have somebody around who she knew when she was a young girl.
I think people often reflect on much older memories after a bereavement, happy times so that could well be when they (the Bartin biys and tgeir girlfriends/brides) were young and relatively care free, when Holly was a baby. Emma was around then, nobody else in Moiras day to day life was. Even Adam can't relive those memories as he is too young.
I've seen people that couldn't stand each other for years ir decades (often siblings) build new friendships at and in the aftermath of funerals due to shared memories and grief.