why would a momma cat attack a kitten

My brothers girlfriend brought her kitten over and my cat who is currently nursing a litter of her own attacked her kitten after she smelled it
And ever since then all she wanted was to attack it
I couldn't even hold the kitten or play with it without my cat getting upset.

We kept the two separate after the confrontation
Since my brothers girlfriend and her kitten were only staying 2 nights at the time

But why did my cat do that I thought cats took in kittens from another litter

How old is the kitten from your brother's girlfriend? And how old are your kittens?

I assume the kitten from your brother's girlfriend is a lot older than your own kittens. Your cat is protecting her litter from the intruder (the older kitten). The other kitten smells differently, and aside from that is probably very active as well. This is a threat to your cat, and thus she goes into a protective mode to make sure her own kittens are safe. This is natural behavior. I would suggest not allowing any other cat into your house again when your cat has her own kittens, unless you really have to. You want to refrain stressing a mom out.

I agree with Bianca above.   

Dont let other cats or dogs near the mom and the kittens, unless you are sure they are good friends.   When a mom is protective, so she is.

Protect her own children is every moms holy duty, no?

My cat had a litter of kittens and she bit and killed her own baby

Mine are almost 2 weeks now
They were less then 1 week when this happened
Her kitten had just been weaned from its momma
And wasn't no bigger than your hand it was still
Quite tiny

lizzy1988 said:

My cat had a litter of kittens and she bit and killed her own baby

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Im sorry to hear it.   Such happens.  not so often, but it may happen sometimes.

1.  If a kitten is sick or dying, the mom disposes often of it.  In nature, she doenst want to attract predators nor scanvengers to the nest.

2.   If the mom is in extrem stress, she may do it being in total panic, totally losing her head.   Happens sometimes yes.

3.  Pure accidents, for example, a young inexperienced mom giving birth, and biting off the cord - misses and if she happens to bite into the kittens tummy, she sometimes continue...

gypsy04 said:

Mine are almost 2 weeks now
They were less then 1 week when this happened
Her kitten had just been weaned from its momma
And wasn't no bigger than your hand it was still
Quite tiny

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See?  Exactly as we guessed.

I suspect your sisters kittens was really too tiny to be weaned and adopted out...   Unless it was a mini-cat, very smallish also as fully adult.

So yeah, such a small kitten would often be adopted by a nursing mom.  This mom choosed to take the sure before the unsure, and did protecteed her own babies.