That's a very odd wording, and I don't think it does what you want. Slivers cast after this can now donate two +1/+1 counters to new creatures.

How about:

> When ~ ETBs, Slivers you control gain graft [for as long as ~ remains on the battlefield].
> Whenever you cast a Sliver spell, it gains graft 1.

I think this could be better worded as:

Sliver cards you own have Graft 1.

or

Sliver creature cards you own have Graft 1.

This is still difficult because they will lose Graft but retain the counter if it dies. The only clean way I can imagine for it to permanently give graft would be one of the above plus:

When ~ ETBs, you get an emblem with "Slivers you control have Graft."

Scratch that, how about:

Sliver creatures you control enter the battlefield with a +1/+1 counter on them and have Graft.

or

Sliver creatures you control enter the battlefield with a +1/+1 counter on them. When ~ ETBs, you get an emblem with "Slivers you control have Graft."

I like the first one in the previous comment. If this dies, you shouldn't be able to keep donating counters. That's not how Slivers operate.

Hey Adak, as it stands, you'd get almost exactly the same functionality in less words from:

Graft 1

Sliver cards you own have Graft 1.

Slick!