The average kitten in a shelter environment is being adopted at 2 months old. If they encounter each other later in life, they will just see each other as cats, not as relatives.
Cats will mostly live solitary lives. In the wild, kittens and their mother would eventually grow to be competitors. So, cats don’t depend on relationship with relatives to survive.
They remember each other via scent. Scent is a cat’s primary way of navigating the world. Since scents can change over time, this could explain why mothers don’t remember kittens and kittens may not remember their mother.
If cats are separated from each other for a week or for a month, it’s pretty likely that they’ll recognize each other. Since we aren't completely able to evaluate feline memory, it’s almost impossible to say how long it takes for cats to recognize their family.
Kittens may look for their mother in her usual spot but that doesn’t mean they miss her in the same way that a human would. Especially if they’re separated at around 8 weeks old when they are ready to adapt to a life without their mother.
If kittens are suddenly removed from their mother, cats may suffer some form of postpartum depression. But, considering the hormones strike, it’s hard to say if she really misses them - at least in the way that humans think of it.