The murder charge alleges that, on July 24 2011, the accused poured petrol, or something similar, through the letterbox of the only door allowing access to the Scott Court home. It was then set alight when the occupants were asleep, it is claimed.
Thomas Jnr and Bridget died in the blaze. Mr Sharkey Snr died of his injuries six days later at Glasgow Royal Infirmary.
Snowden is accused of malice and ill-will towards Mr Sharkey earlier in 2011 by threatening to shoot him and threatening to have his house set on fire.
He has lodged special defences of incrimination and alibi. Between them, the accused face a total of more than 20 charges, all of which they deny.