I have always hoped for a resolution one way or the other in this case. I do not wish to debate the issue as to whether or not Sheila and Katherine are alive today. Everyone is welcome to their own opinion and arguing about it will not determine the truth.
There have been some statements made about how, after 40 years, a missing person is not likely alive. Here is a link to a very recent websleuths thread about a girl who went missing from Pennsylvania in July 1975 at the age of 14, and who recently turned up alive. I don't know many of the details and doubt that it is related to the Lyon case, but missing persons DO turn up alive years later on rare occasions.
LINK:
http://www.websleuths.com/forums/showthread.php?270076-PA-Roxann-Deihl-14-Stroudsburg-July-5-1975