>>42278480 (OP)Both.
In Equestrian culture, older sisters are expected to perform much of the child-rearing duties toward their younger siblings (see Rarity vs. Sweetie Belle and Applejack vs. Applebloom). This is probably doubly so for adopted siblings such as Spike. (It should also apply for Rainbow Dash vs. Scootaloo, but the show makes it clear that RD is pretty irresponsible when it comes to family matters.) A real tipoff is Fluttershy's relationship with her younger yet adult sibling Zephyr Breeze, who still expects her to look after him long after he should be out on his own.