If more than one Sweetsleep is attached to the same character, their constant effects cannot be applied simultaneously. The first player would need to choose which effect applies first, which would then blank the other copies of Sweetsleep. The end result is the same in any case though, the printed text box of all attachments except one Sweetsleep will be blank.
Sweetsleep also blanks the terminal keyword (if it is part of the printed text box), so each other attachment will return to its owner's hand if attached character leaves play.
Blanking the printed text box of cards like Risen from the Sea and Blood Magic Ritual once they become attachments will do nothing. Their current text does not originate from their printed text box, it was granted to them when those events were played, including their Trait and the terminal keyword. Same goes for attachments originating from A Mummer's Farce.