I live in a big flat with my partner. We are both in our early twenties.
I have two kittens and they're sisters from different litters and both female. The oldest is 7 months old and the youngest 10 weeks old. We thought that we'd adopted the youngest at 8-9 weeks old but the vet told us she was only 6 weeks old (I know she should have been with her mother longer but we didn't know until we went to the vets). They are both very healthy (only checked them out at the vets recently) so health problems aren't the issue.
When we got our younger kitten, she was playful and happy even though the older kitten started hissing at her. Now, they're best friends! They cuddle each other to sleep, play together, purr at each other etc, but recently they have been acting very naughty despite me telling them 'No' firmly (method of discipline I use and it used to work).
They have become distant towards us, don't cuddle up to us anymore and the youngest kitten has started showing signs of very aggressive behaviour. If we walk past her she sometimes hisses and growls and has even tried attacking us before a few times (not play fighting). We have not ever hurt these cats or shout loudly in the house so I cannot see why they are starting to hate us (they're our best friend when they want something like food lol).
Also, although they are still the same with each other (playing etc) they have both become very timid. They both cower when we walk past them and have even started going to the bathroom wherever they want. For example; I came downstairs to the kitchen yesterday and the older kitten was peeing on the floor, then I saw the younger kitten run into the kitchen and pee in the same place as well! I have noticed the younger kitten copies the older kitten but I don't want either of them misbehaving!
They are both litter trained (they go 80% of the time and the litter is always clean) and the older kitten was very well trained and was hardly ever naughty before these last couple of months but now they drive us up the wall! It's like the older one has forgotten her training.
I do not know if this is relevant but the older kitten did have separation anxiety and when me and my partner were out (only a couple hours) she'd get very stressed, cry for ages and sometimes pee herself while waiting at the front door for us but all that has stopped since we got our little kitten.
I do know a bit about cats, but I'm very unsure why they are acting aggressive, timid, distant when they never used to be. Nothing is scaring them or has scared them, no changes in routine etc everything is the same as before when they seemed fine.
I just want my playful kitty's pestering us like before :/ now it's like they hate us! Just to make things clear though we have never hurt them or put them out of comfort, never shouted at them or done anything to scare/traumatise them etc, this looks like it has started for no reason that's why we're confused.
Sorry if this is so long I've tried to fit everything in :) thanks for everyone that answers I appreciate your help :)