I guess in my own head I don't think of them looking the same. I think they would look similar like parents or siblings but I personally don't think of them looking identical.

^ This. There are differences in appearance between them. If not physical, then they surely wear different suits.

Superboy has some of Luthor's genes so that could explain any variation. Power Girl is a more adult version, but that doesn't mean Supergirl will grow up to look like that - it's not abnormal for twins to look distinct in adulthood.

^Well I mean Supergirl and Power Girl aren't really twins, but then again they can't be genetically identical since I'm pretty sure Earth-2 Kryptonians aren't powered by the yellow sun.

I know conner kent feels more like lex and superman

"it's not abnormal for twins to look distinct in adulthood."

As an identical twin, I can confirm. My brother and I have a lot of different features. He's slightly taller and slimmer plus he chose different haristyle.

The biggest thing of course is that Power Girl is the only one of the examples that I don’t believe has ever had a backstory making it sound like she was a clone. She has always either had the original backstory of being Kara Zor-L, cousin of Kal-L, the Golden Age Earth-Two Superman or she has been the descendant of the sorcerer Arion of Atlantis. She also is shown a little older and mature than Supergirl even though they are different versions of each other.

Donna also is closer to being a younger twin sister which is a whole different landscape from clone. Only Superboy being in that category. While Power Girl is an alternate earth doppelganger. Meaning different past and circumstance for her physiology and appearance altogether in the decades gap she and Kara have.