^ "It's the girl who saved my life on the day I returned to school following a long bout of sickness, despite us being complete strangers, and went on to become a co-mentor to me in the ways of being a magical girl."
There. A completely true answer which can technically apply to ONE, BOTH, or NEITHER of them.
The oldest child looks so done with her parents, LOL. I like how she's still combing the younger one's hair, she seems to be the responsible type. (^^)
If that's the case, usually I'd expect a bottle of mommy's breast milk(preemptively pumped so Eli can do the feeding). But we're probably putting far more thought into this than the artist ever did.
Given that most everybody refers to the gems as "she", I can understand someone expecting that tag.
But getting philosophical, if we run with the idea that the gems are mankind's "bones", then they would have sexual differentiation of some sort, given the difference between mens' and womens' skeletal structure. Kongou-sensei provides a nice demonstration of this(intentionally or not).