Nonetheless, there are choice-families, and indeed some beings choose to remain living with those who were there when they were birthed/hatched/decanted/came into being. Crechemates might choose to live with each other, and there are group marriages, line- and circle-bondings, and clans of every conceivable type. One common concept is the 'denclan,' a loose grouping bonded by affection, sex, and often (though not always) children genemixed and/or mememixed from existing members; often children of a denclan remain in it, resulting in a high percentage of shared genes and memes.
Incest is for the most part a dead concept, except within certain (generally deliberately anachronistic) factions. There are no medical reasons to prohibit it, and given the nature of Messian life, the potential power imbalance between relations is less a concern; it may not exist, or it may be part of why the pair wants to have sex, and who's to tell them no? Among the wide variety of family titles, 'sisterlover' or 'husbanduncle' would be accepted without even a blink by most native Messians. Outboxers might have a harder time with this, but then again, maybe not.
Some family groupings don't go by any kind of familial titles, and simply use names; some refer to all of their members as 'sibling' or 'cousin;' some delineate those-decanted-before-me as 'aunt/uncle' or 'mother/father,' those-of-my-age as 'sibling' or 'cousin,' and those-decanted-after-me as 'child' or 'niece/nephew.' Some have extremely complicated systems of familial address based on shared genes, chosen relationships, or custom.
Many call their circle of friends their family, and this is as accepted a definition as any involving genes or sex. Many have nothing they would call a family, and that, too, is not especially abnormal.
As touched on lightly above, the concept of children as it exists in a scarcity time -- being limited to specific couplings of a male-female interaction which is then brought to slow fruition within the mother to share only mixed genetic traits of both parents -- is considered unfathomably archaic and quaint, if not outrightly flagrantly crippling the product of such a primitive union.
It is much more common for a single individual, pair, or even a larger family group to take various genetic traits, non-genetic physical aspects (specific materials and enhancements preinstalled), and even copies of memories from the parents or selected from the Paragnostic Desubliminization Pool (usually installed in clearly designated 'elective' fashion, so the child does not confuse them with being its own experiences -- but not always), and create a composite entity, usually with a very short childhood, since ignorance and vulnerability is a conscious choice in all but the most backwards areas of the Mess.
Each warp has its own way of dealing with remixing that puts a unique taste upon the offspring as well. Offspring made by Charm methods will be decidedly different than offspring manufactured through Strange methods...