We have 2 males, neutered in October '12 and 2 females, not spayed yet.
All 4 of the are from the same litter. We think they were born in July.

Problem:
One of male cats is attempting to "mount" one of the females.
If a male cat has been fixed should this behavior be happening? Is it possible for a neuter to fail?

Hi

Its not that long since you had your boys neutered so whilst they cant make her pregnant they may still have some urges from residual male hormones that make them want to mount her. She at the same time could be coming into her heat so she may encourage this. Mounting is sometimes a dominance behaviour in a male over a female so not entirely about mating.They are siblings so will generally play around together more than cats who weren't brought up together. Are you planning on getting the females neutered. I would advise this is the best thing to do anyway.

Cathy Wickenden BVet Med MRCVS
Barton Lodge Veterinary Centre
1 Midland Road, Hemel Hempstead, Hertfordshire, HP2 5BH
Tel: 01442 216048 24 hours
[www.bartonlodgevets.co.uk]