Why is my cat now hissing at her sister?

hi, i took in 2 females from teh asme litter about 6 months ago. they are nearly 6. they have always got on perfectly, sleeping together, grooming each other and followed each other everywhere. two days ago the one - Anoushka, for no reason at all started hissing and growling at her sister whenwver she goes near her. not once has Moomin ever retaliated though. Anoushka now will not sleep near her. she is still fine with us and plays and is eating normally. she has always pulled her fur out (i think as a result of stress in her former home) and this doesnt seem to have altered. the only thing i can posssibly think is that the day is started Moomin was out rolling around in the garden while we cut the gr but Anoushka wasnt. they have both been out on the gardne before but not when the gr has been cut. could this have anything to do with it and if not what do i do?!