There's a marriage of George Sinclair to Margaret B Black in Glasgow Blythswood in 1914. Go to
www.scotlandspeople.gov.uk, invest in a few credits and use some of them to buy an image of the marriage certificate. It will tell you the names of George and Margaret's parents including their mothers' maiden surnames.
Margaret Bell Black, other surname Sinclair, mother's maiden surname Bell, died in Port Glasgow in 1975, aged 83. This could be Pearl's mother. She will be ther Margaret Bell Black born in Greenock West in 1891.
There are 12 deaths of Sinclairs with mother's name Black born 1925 plus or minus 10 years. One of them is Elizabeth Sinclair, other surname Lindsay, who died in Greenock in 1989 aged 68. However the mother's maiden surname is only included in the 20th century indexes from 1974 onwards, so any siblings who died before 1974 would not show up in this search.
Elizabeth Sinclair and John Lindsay were married in Greenock West in 1942. Their marriage certificate, available from Scotland's People, will tell you if this was your Bessie.
Could your George Alexander Sinclair be the one who died in Greenock in 1950 aged 62? His death certificate, also available from Scotland's People, will tell you.
Any idea what age and sex the sibling was who was killed in the tram accident?