We need to denote a relationship between A and B in column C once we detect that inverse of values is also present in some rows, where A becomes B at some row.
Is this possible in Sigma?

This is tricky but this workaround might work out for you! You get close to what you are looking for by taking this table as input 1, and left join it again as input 2 with the conditions (input1.a = input2.b) and (input1.b = input2.a). Then you will have to set column C to “IsNull([input2/a])”. This should work out for you but there will be some duplicates, so you need to try and de-duplicate any of these values you see!