Strange Cat Behaviour

[Winter&willow]

Hello!
I have two 1 year old kittens who have lived with each other since birth. All of a sudden, one of the kittens, Willow, has started acting strangely around the other kitten, Winter. Winter is acting the same as usual however Willow is constantly sniffing Winter and approaches him very slowly. If Winter looks at Willow she starts retreating. This is unusual as they normally act friendly towards each other. This has just started happening so it might be nothing however, I'm worried this might get worse and they wont be able to be together.
Does anyone know what is happening or have a solution of how to make them friendly again?

Thank you!
Winter and Willow x

 

[scraggles]

I am by no means an expert - but just from the bits I've picked up on this forum - are the by any chance opposite sexes and have they both been neutered yet?

 

[Winter&willow]

Thank you for your response. Yes they are opposite sex and they have both been neutered.

 

[scraggles]

Hopefully someone more knowledgeable will come along as I just wanted to double check that they're neutered and am afraid I can't add anything much more constructive...

I'm hoping it's not serious

 

[elliesvictim]

Sounds like they're growing and figuring out the pecking order, sorting out territory and boundaries.
They're not howling, hissing or fighting, so I wouldn't read too much into it.