In what way is Cybill Shepherd related to Penny Singleton?  They look
so much a like.

I have never seen anything to indicate that these women are related to
one another. There were many articles about Penny Singleton at the
time of her death last year; if she had been any relation to Cybill
Shepherd, I would think someone would have mentioned this.

A facial resemblance doesn't necessarily indicate a close genetic relationship.

When Orson Welles appeared on MOONLIGHTING, he personally requested
the best cue-card man in the business to help him with his lines,
Bernie McNulty.  Bernie was Penny Singleton's younger brother.  Other
than this there appears to be no connection between the two actresses.