Most casinos use the rule that only a full bet (at least as big as the last raise in that round) reopens the betting to a player who has already acted. In this case SB may not reraise here. This is the rule used by the Tournament Directors Association.

I know of one casino that uses the rule that a bet that is 1/2 (or more) of the last raise reopens the betting (this rule is pretty much the standard rule for Limit)