Hi Ed, They're invisible for three reasons. First I'm pretty sure that Barbara did marry since she had two daughters. You'd have to find her married name to find her obituary.
In 1930 she was living in Manhattan, NY so we don't know if she married in New York or Alameda County California. The 1940 Census will be issued next month and when it's indexed, you'll know more about her.
The other reason they're invisible is because they're probably still alive.
The third reason is because we don't know their married names.
I did find Barbara in the SchumanGeganBruendermanSchirrmacherRittmeyer 10-2011 family tree, trees.ancestry.com/tree/9506534/person... However it doesn't have her married name and just shows her death info as 15 May 1988, Alameda Co., CA.
You can ask the Alameda Co. obituary volunteer to look up her obituary but I don't think she'll find one. The volunteer can be contacted at freepages.genealogy.rootsweb.ancestry.com/~obitl/volca.html Successful Searching!