Is it strange to have one mother cat take on a 2 week old kitten with her newborns?

Long story short my cat Momma had a litter of kittens two weeks ago. Out of a litter of 7, six kittens died. I don't know how. Well Momma brought the baby out and just left it and my other cat pebbles who also had a litter of kittens two days ago picked up the kitten and took her in as her own. Is this normal? I'm also concerned because this 2 week old seems to be weak and also has a soft, squishy lump near her spine on her back which just appeared this morning. Any ideas about the lump and is it safe for Pebbles to take in this baby kitten which isn't hers?

Since the kitten isn't from outside your household then yes, it's probably safe, but the vet needs to see the squishy lump.  You also need to get your cat that lost the babies neutered - she will get pregnant again very quickly - and the other girl once she has weaned her kittens.  She might start calling while feeding them, so keep her in until she's neutered.

Really depends why the others died, and why this one appears to be weak. It may have something that could be passed to the other litter, that could also be why the mother has rejected it, instinct tells them to leave the ill behind.

The soft lump is concerning, please have a vet see him. 

I read in a cat book i got from the library that if a kitten is rejected by mum they can be fostered by a new mother as long as its soon after birth she supposedly wont know the difference between it and hers. It doesnt say what it means by "soon" but it looks like your other cat has decided to play mum, so i would guess that it is fine.

But tbh im a novice kitten owner so im sure the others know far more than me lol!

Hope the lump turns out to be nothing serious, thats something i dont know much about sorry >_<
 

There is no issue with the other momma taking in this kitten, however, I would get the kitten in to see a vet ASAP. The original mom rejected the kitten for a reason (there are bad kitty momma's that just won't take care of a litter though, rare, but it still happens).

I wouldn't worry about the other momma taking in the kitten, she chose it, you didn't try to force it on her so she will be fine with it. It's actually better for the abandoned kitten to be getting kitty milk and the socialization from being with other kittens. It's really a big win for the baby.