în this universe everione gets a mythical species.The commone ones are cat/dog,rare ones snake/bee,legendary whitch were demon/angel and the mythic one whitch was siren.Being a siren was very rare and less than 0.01% of the population were sirens.After your twin mean sister got her angel form,you realised you didint had one(at least you thaught).Nowhere was specified that once you get siren,you have to be in contact with water to get your form,so you thought you didint have one,but u had one