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.