Orphan Black: confused, science/medical question

[AdultingIsNotWhatIExpected]

Please no season 4 spoilers, have only watched 3

why can't Sarah or Helena be Cosima's donors?

Something to do with them only having half of the genomes/ original genomes I think it was. Or just to create a plot point?

They don't have her disorder and would be a 100% match
Surely ideal?

They have a full set of genes, they're clones of someone with double genes, they have the girl line, which = the same amount of genes as Kira but Kiras are a mix of Sarahs and her Dads

Yes, but they need the original genome for the cure. They did use Kira's stem cells (sorry, not sure if it's a spoiler or not, can't remember now, so apologies in advance), but they didn't work. They need the original genome that had both the male and the female genes and the code that allowed for them to be cloned.

I do have to say that the further we go on, the harder it is becoming to follow the plot/ science behind it all.

That's not what I mean, I mean when they used Kiras to treat/slow it, why now Sarah's/Helenas then

Kiras stem cells did work, they weren't supposed to be a full cure, but they did help/treat.

Why was Kira ever used and not Sarah?

I honestly can't remember now why. Perhaps because Kira had that healing superpower?

I found this article, but it doesn't seem to mention why they didn't use Sarah or Helena either books.google.co.uk/books?id=v0UFDAAAQBAJ&pg=PT45&lpg=PT45&dq=cosima+illness+cause&source=bl&ots=iGpA8j3LN-&sig=xgSKyf5E31hQ2oD45Y3PuB2tcUY&hl=en&sa=X&ved=0ahUKEwjypaKSmJLOAhVKlxoKHWpWCVsQ6AEIOzAF#v=onepage&q=cosima%20illness%20cause&f=false