Missing siblings

I took a trip to my great grandfather and great grandmother`s grave in Philadelphia and saw two other gravestones, for two people, who presumably are their children. I had heard of the two "ghost siblings", as 'Doc" & "Maggie" - and the sibling graves I found were Stanley Schubert, and Maggie Schubert. Not married, defiantly siblings, and children of my great grandparents. I cannot find ANY matching records for these two. Can anyone shed some light on this? Thank you.

STANLEY SCHUBERT
1885-1956

&

MAGGIE SCHUBERT
died 1970

Buried at Holy Cross Cemetery, Yeadon PA.

They would have lived in Phildelphia.

Thank you!

Missing siblings