Mother cat "abandons" 2 of 5 kittens?

Hi, We have been feeding feral cats for some time now and more recently
trapping and neuter/spaying them.
Four weeks ago a mother had 5 kittens in the garage.
Near the time they were able to walk she relocated them under a
neighbors house. I expected to see them again when she brought them
over to eat solid food.
However, a week later she brought two of the five back to the garage.
They seem quite healthy and took to solid foods (@four weeks) . I'm not
sure but i don't think she is paying any attention to them at all.
The thing is,I see her nursing the other kittens 15 feet away on the
other side of the fence.
I suppose the little male and female are cute and tame enough to be
adopted to a good home.
Any thought? You wouldn't try to reintroduce them to the
litter would you? Is it correct to find homes for kittens in this
situation? They come to us after on two days.
Thanks Rob