I propose the following:

Aliases:
brother_and_sister -> invalid_tag
father_and_son -> invalid_tag
father_and_daughter -> invalid_tag
mother_and_son -> invalid_tag
mother_and_daughter -> invalid_tag
grandfather_and_grandson -> invalid_tag
mother_and_father -> invalid_tag
father_son -> invalid_tag
Reason: Searching for brother sister will return the same posts that brother_and_sister would. Thus, such tags have very little value. On the other hand, brothers would be harder to distinct with just a brother tag so I'm not proposing to invalidate brothers and sisters.

Implications:
son -> child
daughter -> child
Reason: Considering father and mother imply parent, son and daughter should also imply something.
Please note: child is currently already used to indicate age. So.. would it be more correct to use a different name here, or to first rename child to something else? Or perhaps disambiguate them as child_(age) and child_(family)?

More implications:
child -> family
parent -> family
sibling -> family
Reason: An umbrella tag for First-degree relatives It was already proposed before.. somewhere.

family -> relatives
Reason: An umbrella tag for any relatives.

More implications:
grandfather -> relatives
grandmother -> relatives
grandson -> relatives
granddaughter -> relatives
aunt -> relatives
uncle -> relatives
niece -> relatives
nephew -> relatives
Reason: An umbrella tag for second-degree relatives.

Alias:
cousins -> cousin (or vice versa)
Reason: Unlike brother, it's impossible for only one cousin to appear on a post. So the distinction between cousin and cousins is impossible.

Implication:
cousin -> relatives
Reason: Third-degree relatives. Considering there's no great grandparent tags, I don't think we need to implicate those yet.

More implications:
wife -> spouse
husband -> spouse
spouse -> family
Reason: An umbrella tag for family-in-law.

Alias:
step_mother -> invalid_tag
Reason: Not enough value to tag stepfamily.

TLDR:

• relatives
• family
• parent
• mother
• father
• child
• son
• daughter
• sibling
• brother
• brothers
• sister
• sisters
• twins
• triplets
• spouse
• husband
• wife
• grandfather
• grandmother
• grandson
• granddaughter
• aunt
• uncle
• niece
• nephew
• cousin

You are correct that brother_and_sister is defined by the wiki to mean simply a brother and a sister, which is equivalent to searching for brother sister. This use of the tag is pointless. However, users should be able to search for a brother and a sister interacting sexually, the same way they can search for a male and a female interacting sexually.

I propose that we change such tags to brother/sister to match the male/female formatting, and sort through current brother_and_sister posts to change them into either brother/sister or brother sister.

...Actually, a search for brother sister incest is almost identical to my proposed brother/sister tag. Maybe we don't need it at all.

leomole said:
...Actually, a search for brother sister incest is almost identical to my proposed brother/sister tag. Maybe we don't need it at all.

... was about to write that. -1. I don't see the need for such specific tag combinations when incest covers all sexual intercourse among relatives.

Also, this thread currently has nothing to do with incest..

at least not until I propose the implication
incest -> relatives
:P

leomole said:
...Actually, a search for brother sister incest is almost identical to my proposed brother/sister tag. Maybe we don't need it at all.

If you search that, there are likely to be several images in which the brother and sister are not necessarily interacting with each-other; for instance:

The image has a brother and two sisters, but Big Macintosh, the brother, isn't directly interacting with either. One could make the case that adding duo would fix this, but that would be excluding images like this:

In conclusion, I think that the brother/sister, or father/son tags are quite helpful for those whom like incest, and should be used for their convenience.

The post which you linked is falsely tagged with incest, because none of the related characters are directly interacting with each other.

But to help you out, I searched all the incest posts. I was only able to find 1 post, where brother and sister were not directly interacting. So your argument in favor of creating a crapload of <relative>/<relative> permutations isn't exactly.. convincing.
In other words, the search string <relative> <relative> incest works correctly in at least 99% of the cases, so it's more than adequate. It might have been a different story if the search string would miss certain posts, but that isn't the case as it returns all the posts, albeit with a couple of false-positives.

I was under the assumption that two siblings within a sexual setting, while not necessarily interacting with eachother, was still incest. I seem to have been mistaken, based on it's page. I kinda always considered situations like the image below to be incest:

Shouldn't they be aliased to family instead of invalid_tag?

Nope.

The tag family should never be used by itself. It should always be included purely through implication from the more specific tags, brother, sister, father etc.

The correct thing to alias these tags to would be two included nouns. But since that's impossible (alias can only point towards one tag), it's better to just tell the tagger that the tag is invalid, and that he should use the two nouns instead.

At least that's my opinion.

Delian said:
Nope.

The tag family should never be used by itself. It should always be included purely through implication from the more specific tags, brother, sister, father etc.

The correct thing to alias these tags to would be two included nouns. But since that's impossible (alias can only point towards one tag), it's better to just tell the tagger that the tag is invalid, and that he should use the two nouns instead.

At least that's my opinion.

I disagree. It seems better to me to actually have a valid tag than an invalid one.