S1 says Sohan's weight is smaller than Mohan's or Rohan's (since twice Sohan's weight is still less than theirs). S2 says Rohan's weight is greater than Mohan's or Sohan's (since twice Rohan's weight is more than theirs).

From this: Sohan's weight is the smallest. Rohan has the largest weight.

So, the correct answer is B. Weight of Sohan is least.