Translation:My uncles are twins, but they are different.
So they're fraternal twins, not identical twins. Is there a way to differentiate these two kinds of twins in Esperanto? Frata ĝemeloj kaj identa ĝemeloj, perhaps?
Well, the English Wikipedia also talks about "monozygotic twins" versus "dizygotic twins", but that doesn't mean that many people use those terms or even know them.
Those are scientific terms, but most people that I know use "identical twins" and "fraternal twins".
For what it's worth, PIV has s.v. ĝemelo the examples ĝemeloj unuovolaj, duovolaj. (I haven't heard those, either - but then, I haven't talked about twins much in Esperanto.)