I am writing about domestic cats. The reason for that is because not infrequently there’s more than one female cat available to share the responsibility of raising kittens. In the wild, it is far less likely that a female cat will encounter another and therefore what I describe below applies to domestic cats.

RELATED: Mother cat engages the help of family dog to raise her kittens
The answer to the question in the title is yes, she will. It’s a known fact that if a nursing mother has a normal-sized litter it is possible to add one or two orphaned kittens to it without much difficulty. The mother will usually take care of them as her own.
We’ve often seen this in videos on the Internet with some nursing females being only too happy to look after kittens simply introduced to her meowing and ready to be nursed.
This is because her maternal instincts are very strong. She’ll be unable to resist the calls of the kittens for help and will normally approach them, pick them up in her mouth and place them in her bed.
She’s likely to lick them and her scent will be passed to her kittens which will allow them to feed alongside her own kittens.
I’m told that some breeders make this process work by giving the mother cat some assistance. They wait for the mother to leave the nest and then take the strange kittens and rub them gently in the bedding that carries the nursing female’s scent.
When she returns, she relates to the strange kittens as her own. It seems that mother cats don’t count their kittens. When the strange kittens have the smell of her own kittens there certainly isn’t any difficulty.
Observers have reported that when there is a large number of female cats together in a cattery, kittens are often shared out between the mothers.
The mothers show a remarkable degree of social tolerance. Sometimes they reside in large communal nests and kittens are brought there.
It’s been observed that on one occasion six females with 18 kittens were in a communal nest of this type and each female allowed the other mothers to nurse any kittens whenever they wanted to.
Normally when a kitten is being nursed, they establish ownership of one nipple because of the scent from the nipple and it helps to ensure peace among the feeding kittens. But in these nursery nests the kittens took the first nipple that was available and such “nipple ownership” did not occur.
It is said that this nursery situation results in strong and healthy kittens. The drawback is that if a kitten is a runt or weaker than the others, they might end up at the bottom of a pile unable to breathe.
As mentioned in the first paragraph, this kind of communal female nursing behaviour would never normally occur in the wild because of the large size of the female cat’s territory preventing them to encounter each other. There will be little or no pressure from an evolutionary standpoint for this sharing to take place or for mother cats to develop and amenable reaction to strange kittens.
Blood type incompatibility
There is an interesting question from a reader on the Pet Place website. They say that they had two adult female cats who each had five kittens and they were born two days apart. One of the female cats kept on taking a babies to the other cat resulting in the other female cat nursing 10 kittens. She was asking whether this was a problem.
The problem is that one female cat does all the work and it might be a drain on her. Further, the response to the question is from a veterinarian and they say that there is a risk of a condition called neonatal isoerythrolysis. This is when a mother’s milk is incompatible with the blood type of the feeding kitten. This is because the mothers colostrum contains antibodies from type B blood specifically created to destroy type A blood. If the kitten has type A blood it can result in serious illness and death. Most kittens under the circumstances die within the first 48 hours of life. (ref: Dr Debra).
In the circumstances described it was probably okay as the mothers were sisters.
RELATED: What blood type are cats?