Initially I was going to ask a question about how one should determine whether to tag selfcest or tags like twins, triplets, etc. when there's no explicit confirmation in the image, but as I started thinking about it the scope of it expanded.
Incest is a lore tag. There is no non-lore version of incest for images in which the familial relation between two parties is explicitly confirmed within the image. This seems sensible enough to me, so long as one can assume that in-image text isn't taken into consideration for the tag what you see policy.
This becomes a problem though when one looks at most other relation tags. Sibling, son, daughter, brother, sister and mother are all regular tags, not lore tags. Even selfcest, which is directly comparable to incest, isn't a lore tag. These are all in the same boat as incest, where the best you can determine by looking is "those characters look similar, they might be blood related". Skimming through some of these tags it seems that many images lack any kind of confirmation of the subjects being related, some (such as the majority of the results for the two characters in this search: https://e621.net/posts?tags=amalia_%28claralaine%29+ken_%28claralaine%29+-siblings) don't even look similar enough for me to be able to tell they're siblings without looking at the character tags. Not even twincest, which is just a form of incest, is a lore tag.
It seems to me that, for the sake of simplicity, all family relation tags should be changed to lore tags. Identifying them in isolation is a guess at best, and it seems unnecessary to split the tags into two, while the existence of images where canon family relations exist (and especially ones that can be determined with a simple look at character tags) means lore tags are necessary. Some people are more or less comfortable with certain forms of incest than others, so at the very least for these people the relation tags are necessary to filter specifically the kinds of incest that make them uncomfortable out of searches. I personally know people who are comfortable with sibling incest, but not parent incest, for example. (Mainly people who don't have siblings). Without lore tags for family relations its impossible to keep this nuance consistently and follow TWYS guidelines, and keeping family relation tags as regular tags that are subject to TWYS also opens up the possibility of similar looking characters that aren't blood related being tagged as if they were, and I imagine that should be avoided as well.