okay, done some fishing for you on Scotppl site and this is an educated guess, but you may be able to dig deeper if anything rings a bell.
Checked for Margaret or Maggie Webster's born in P'head. Found 1, Margaret Watson Webster born 1910 who died in 1975 age 66. Other names listed 'Milne'.
Looked for births and found Margaret Watson born 6th Dec 1910 to John Webster (stonepolisher) and Elizabeth Webster nee Crowe(?) address given 36a kirk street p'head.
ref for birth is 232/01 0471
No way to be sure of the missing sister, as I cant check the parents names v helen's birth as the record has to be ordered and cant be viewed online. May give you a start though
Margaret Watson Webster.