A Genestealer Familiar is a weird, WEIRD type of Hybrid variant used by Genestealer Cults. These tiny figures are not truly creatures of flesh, but rather constructs of the psychic might of a Genestealer Magus. Think of them as similar to the Haemonculus from Full Metal Alchemist, artificial robots made from organic flesh, blood and bone designed to accomplish one or multiple things at once.
Should a Patriarch or Magus of a Genestealer Cult dominate a region completely, the psychic shadow that gathers around him will grow darker and thicker until it begins to coalesce. This ever-shrinking miasma hardens into a many-limbed form known as a Familiar. When the familiar is fully manifested, it skitters from its hiding place and runs to the heel of the xenoform that unconsciously created it. From that point on, it is as much a part of him as any other body part. Small and nimble, such a familiar is an excellent spy, able to scuttle through the tightest crawlspaces and report back to the cult’s leader through the potent psychic bond that links them.
Familiars are probably used to help assist a Magus in both combat and the further spread of the Cult, and is sometimes used as replaceable meatshields in times of conflict. How or why the Genestealer Magus decide to make what counts as organic robots is unknown. The process in the creation of a Familiar is a even bigger enigma, with some in /tg/ speculating that the Cult might have 'stolen' some Haemonculi technology from the Dark Eldar.
On tabletop, for 12pts you get an extra body to add onto your Patriarch or Magus. The Familiar has the same statline as an Acolyte Hybrid, albeit with no weapons, no BS, +1Ld and a 6+ save (rather than a 5+). Once per game if your Magus or Patriarch has at least one familiar, you can cast one additional power - this will almost always be Smite. As it counts as a separate model, it can also absorb one (or more if you're lucky) hit which would otherwise hit your Magus/Patriarch.
Tl;dr, its a psychic meatshield. Abuse it as much as you can.
A sub-species of the regular Genestealer Familiars. Alchemicus Familiars are beings made specifically for the Genestealer equivalent of a mad scientist called the Biophagus. Essentially this cooky little guy with the goggles carry around its master's concoction of gene-edited liquids to forcibly turn unwilling 'volunteers' into sleeper agents and servants for the Genestealer Cult. Although seeing as how most Familiars lack eyes, it is unknown why this Familiar requires goggles in the first place.
The creation of these Familiars should be very similar to their more common brethren, which is basically the condense psychic energy being coerced into a physical being of the materium. However, it is unknown if the Biophagus himself is a powerful psyker or it is a result of some much added teamwork. It could also be that it is nothing more than a gift to the Biophagus from either the Patriarch or Magus.
Crunchwise, the Alchemicus Familiars are primarily a synergy unit with no actual combat capabilities. It's just there to keep its master happy and convenient; a lot like the Watchers in the Dark honestly. The Alchemicus Familiars allows you to roll 2D3 on the Genomic Enhancement and pick the result. It’s only once per battle but that can help add some consistency to the roll and really make that one unit of Aberrants perfect for the job you need them to tackle.
Essentially speaking, the Igor to the Biophagus's Dr Frankenstein.
Another sub-species of Genestealer Familiars. Mindwyrm Familiars tend to find themselves attached to Abominants, the massive hulking alpha-aberrants within the cults. These are generated by the Patriarch to convert a chosen Aberrant into an Abominant through an infusion of the Patriarch's biomass; after the new Abominant is formed, the Mindwyrm stays with it to ensure that the Patriarch can do all the thinking for the big lug.
How the Aberrant can be upgraded is similar to how the Xenomorphs can upgrade a normal Drone into a Praetorian: through special hormonal means. The Familiar implants the selected Aberrant with a portion of the Patriarch's own biomass with an organ similar to a Genestealer's Kiss. How something so small can reliably 'get in there' would be a scene of absolute degeneracy and obscenity. Imagine a midget french kissing Andre the Giant.
Once the injected mutagen is implanted into the chosen Aberrant, the creature is reshaped over a long and agonizing process. Its already robust frame grows even larger and more powerful, while its bones break and are reformed into a more powerful skeletal structure. It gains new potent regenerative abilities that help it survive this process as well as endure in battle.
A Mindwyrm can be differentiated by its more scarred face and a bitchin cloak.
Unlike the other familiars, these guys are not considered separate from their Abominants. They offer 2 extra S4 AP- D1 attacks for the Abominant in melee and force opposed non-Tyranid psykers within 12" to take -1 to their psychic tests.
These subspecies of Genestealer Familiars tend to be joined to Sanctii and act as spotters for the assassins.
Like the Alchemicus Familiars, these particular familiars are identified by their multi-lensed goggles, which is rather meaningless for something without eyes. Instead, these familiars act as the Patriarch's eyes and enable them to direct their assassins' weapons. These goggles may or may not be 'gifted' from imperial munitorums and factorums, most likely helped by either an undercover Cultist or even the Familiar itself being extra sneaky.
As they are formed from the psychic condensation of a Genestealer's Magus or Patriarch, the Soulsight Familiars are a 'gift' to the Sanctus that is both to check the surroundings and aid the Sanctus in his hunt for his prey. Whether these creatures are made of biological flesh and blood or beings similar in nature to a Daemon is unknown. But the idea of a Tyranid Daemon is a pretty cool idea in that regard.
Other than the goggles, the Soulsight can be distinguished by its long and perverse tongue. For all intents and purposes, they are the Genestealer equivalent of the Ork's Targeting Squig; as they are both biological git findas/spotters.
In crunch terms, this allows a Sanctus to ignore any cover when they make an attack. This is on top of it's own extra 2 S4 AP- D1 attacks that it makes since it's on the same base as the Sanctus.