Any ideas apprecitated on this guys.

Bought two identical sister kittens in Novermber. Both been very good using
litter trays as they are supposed to until last week.

Now one of them has started doing it everywhere but in the litter tray and
even on the bottom of the curtains. No change in litter and she even buries
the other kittens poo in one of the litter trays so she knows where they are
etc.

Just to let you know we have two litter trays neither of which have been
moved or cleaned with any chemicals.

Appreciate your help guys. At my wits end.

Gareth