^ that's basically it. we need external sources to establish relationships. it can't be deduced outright by looking at the image alone. original siblings chartags:0 return results. or in cases where the artist placed a name into his original character: original siblings chartags:2 also return results. edit: *this is a response particularly about reliance about outside sources, e.g. the author's words.*
however, i see no harm in them. original *_and_* spoilers and original siblings spoilers , interestingly return zero results. if somehow we limit these family tags to original/non-series personalities and be able to survive for more years to come, then i wouldn't mind, since they're unlikely/remotely to carry spoilers. maintenance by then will be limited to trimming down family tags posts to no known copyrights aside from being original. work load reduced by ten-fold.
but then again, i don't see the necessity to mirror all the tags used by the author and how could it exactly help the user filter his searches.
looking at post #320736 , what makes the brother_and_sister tag special? there's nothing to indicate in the picture that they're siblings, just the tag. i could probably use 1boy 1girl motorcycle original (this fails) or chartags:2 motorcycle original (works but questionable use of chartags). or alternately since the characters are named kanzaki_hayato kanzaki_miku motorcycle (works fine).
regarding unnamed characters: original family chartags:0 return some results. as Serlo pointed out, this might be a pool material. we might redefine (if applicable) some pools to accommodate such or create new ones. possible i think are slice of life, family-bonding, or heart-warming if they're not part of it yet. the filial relationship between characters are just auxiliary information, better left unsaid or off-site.
post #1114534 is old_woman kids, no need to indicate if the kids are her grandchildren or some kind young neighbors visiting a childless widow. i can't say for other posts at the moment because some are lacking the necessary tags without adding them first. but if they are not pool material, i can say most of them can be searchable by tags without family tags.
post #906084 is most probably a pool material. the taller girl can be her mother (if only it's not tagged as siblings) and can be searched using 2girls child holding_hands .
in any case, we are just exhausting every possible and available means of locating a post without using family tags. if there's one post that can't be located without using a family tag or not a pool material, i'm very open to know an example of it.
EDIT: *after reading Log's edit ^^^ above ^^^* i checked original spoilers. the sole entry has nothing to do with family relationships. i used original siblings spoilers or original *_and_* spoilers to demonstrate/get posts that has something to do about close associations which is relevant in what we are discussing - original/unnamed characters from non-series copyrights. the * is for father, mother, etc (instead of typing each and every one). however since original spoilers only has one entry and that post contains no family tags, my 2 searches obviously will yield zero post.
EDIT2 : @S1eth *vvv below vvv*
saigyouji_yuyuko 2girls child should work. but since the guy we are interested in is a recurring character in one artist's work only, maybe shin'en_(gyokuro_company) saigyouji_yuyuko child is fine too.
if we want the "mother and daughter" atmosphere alone, without the "fatherly" presence, maybe saigyouji_yuyuko 2girls child -1boy -faceless_male (though this not exactly work because of some missing tags; edit: tags added)
do these images of saigyouji_yuyuko mother_and_daughter count because the relationship is visibly apparent?