interprogeny means between progeny. Lexicurio rates it Rare gem — a strength score of 82 out of 100.
Why “interprogeny” is a great word
INTERPROGENY — [Adjective] Existing or occurring between the offspring of a parent or parents. From the English prefix inter- ("between") and progeny ("offspring, descendants"). Unlike intergenerational, which bridges the chasm between distinct ages, or intrafamilial, which blankets the entire household, interprogeny delineates the strict, horizontal axis of those who began from the same source. It is the secret language in a darkened room, the inherited quarrel over a parent's affection that outlives the parent, and the silent, comparative measuring of lives launched from the same shore—the foundational and forever tension of those bound by a common origin.
Etymology
From inter- + progeny.