-Belos didn't bother with Darryl. Molly was compatible, but her power was too weak, so Belos figured it'd be the same with her brother. (Not to mention there was too much of a gamble in that scenario. One sibling was enough, but a second one would raise suspicion).
Marco is the more ideal candidate because he survived a LOT and has a bit of the cadou inside him thanks to Bill (which was done as a sort of backup 'screw you' to him and Star.)
-I think Mariposa was a sort of 'at the wrong place at the wrong time,' and the girl was none too happy to be seperated from her brother when he was crystalized (and... ah... put in four flasks, just like in the game.)
Eclipsa called dibs on Mariposa really fast, and Belos allowed it since it wouldn't have made much of a difference and had a plan for getting rid of the lords since he didn't need them anymore...
Mariposa was safe at Eclipsa's at first, and even befriended Meteora. But shit went down, Eclipsa got crystalized by Belos for failing to stop Star, as did Globgor as part of the punishment, and Star was disturbed by those events. Meteora was spared (since she didn't have a cadou inside her to activate the specific crystalization process), and both she and Mariposa were entrusted to Papa Titan (along with the crystalized Eclipsa and Globgor) for safe keeping.
....Oh yeah... the crystalization thing from the game. I'll talk about that in a separate post. You don't even have to ask about that one because I finally figured out how to bring that into the AU. And... let's just say I know some people are not going to be happy with me.