Seeing as Luke and Leia as twins was a last minute decision in the eighties when the duo's career was already winding down and came out the year Karen died I can't see the connection.
There is more connection to Carrie and Karen through Leave Yesterday Behind than between Leia and Karen through Star Wars.
Indeed I see more of a connection between their cover of Klaatu's Calling Occupants of Interplanetary Craft and Close Encounters of the Third Kind.
The time scale is roughly the same but at least the subject was the same.
It's probably unfair but the general perception of the band is that Karen was the band and Richard was the guy in the background barely if ever noticed.
You can't really apply this to Luke and Leia, where Luke arguably grows to completely overshadow the character finally and conveniently revealed to be his sister.
Show people a picture of Richard out of context and most people (even people around at the time) would struggle to identify him. The same can not be said for Karen.