i understand your concern, however, does he make HER happy? b/c if he does, itll give her another reason to keep fighting. Trust me i keep telling my dad, just because i have C.F doesnt mean i dont wanna have fun still i dont wanna be happy. i wish i could find some one to share my shorten life with. but everyone i tell i have Cf too they run away. instead of not standing by her let her be happy. he didnt run did he? do ya no how many guys would have? he obviously loves her. support your sister. all of us sick or not sick has 1 live to life. let her live her life. shes 18 like u said, shes old enough to make the right decisions for herself and if she doesnt she will only learn. but you should be happy for her to find a guy whos willing to stand beside her and not run off.
best of luck to ya and your sister