Hello and welcome to Rootschat!
Do you mean Mary was buried on 9 Feb 1752?
If you haven't already got them, FindMyPast and Ancestry have two other Siviers who marry at about the same sort of time in N Stoneham, so could possibly be siblings of Robert. Joseph married Anne Baldrey (licence 4 sep 1733) and Elizabeth Sivier married Matthew Huelin 14 July 1726 (licence 13 July 1736). FindMyPast has burials for a couple of children of Joseph and Anne's at North Stoneham.
Unfortunately, no sign of baptism for Robert or the other two, which I guess would be about 1700.
Sivier sounds as if it could be French - could they be Huguenots? I have Huelin ancestors, mine are from Jersey - possibly Huguenots.