No.. Judy Evans and Mary Beth Evans are not related... Here is the link to an interview asking the same question...
"Days" news. Here is a reply from Judi Evans from a Soap Opera Digest article in 1999:
Question: "When you were on Days, we thought there was a resemblance between you and Mary Beth Evans. Are you sisters or cousins?"
Judi Evans: "Not even close! Nothing! Everyone thought that, but no. Blond, blue-eyed girls from California, but that's it. As far as we know, no, we're not related. But
thanks because I think she's very beautiful,so to say we look alike or somewhat alike-- that makes me feel great, thanks!"
Source: 9/21/99 Soap Opera Digest Online