Sir Alan Durward - Are his daughters the same person?

Sir Alan has two daughters N.N. Bisset (Durward) and N.N. Durward that are half sisters. Both married a John Bissit. Both had a daughter Elizabeth that married that married Andrew de Bosco and Andrew de Boscho. They had daughters Maria and Muriel that married a David Graham of Lovat. It just seems all the names and dates this close together that they may be one and the same. If they are there are several profiles that would have to be merged. If all of them are correct it just looks strange to me?