Sno was named on the bench in the Boxing Day defeat at QPR despite the recent bereavement but he will now fly home for the funeral and is therefore unavailable.
Gary Johnson almost missed the trip to Loftus Road with a chest infection and a similar illness kept his son Lee out of the squad on Saturday.
Midfielder Johnson is unlikely to be involved again as he continues to struggle, although Marvin Elliott could be in contention for a start after he was deemed fit enough to be on the bench at the weekend.
The Robins find themselves on a poor run of form having won just one game in the last nine and without a home win since October.
Watford duo Henri Lansbury and Heidar Helguson will play the final game of their loan spells.
Arsenal youngster Lansbury and QPR striker Helguson are due back at their parent clubs after Monday's trip to Ashton Gate.
Boss Malky Mackay hopes to extend midfielder Lansbury's stint from the Gunners until the end of the season but the club are unlikely to be able to afford the wages of Helguson, who has scored five goals since rejoining the Hornets.
Another loanee, Manchester United defender Craig Cathcart, has returned to Old Trafford for knee surgery and faces a spell on the sidelines.
Teenage midfielder Ross Jenkins is Mackay's only other injury absentee, having been been sidelined since October following a back operation.
Sporting Life