Lots of people find that the behaviour of other pets in the house alters after losing one of their companions. This may be that they directly feel upset – the dynamic of the house has changed and they will be able to sense the stress and emotion in the house. They may feel grief themselves, this is something that we don’t yet know enough about. But you might observe crying of the remaining pet(s), wandering and appearing to be looking for the departed pet, oversleeping, changes in toileting routine or mishaps around the litter tray or reduction in food intake. Departure of a pet from the group will also change the group hierarchy and this will need adjustment to. Find out more here.
Keeping to routines will help: keep diet and meal times the same, walk as consistently as possible. In time their grief will diminish.
Some people allow their remaining pets to see and spend time with the deceased pet, in an effort to ease their distress. Evidence for this helping is anecdotal at this stage, but it may be something that you wish to do.