Why is my older male cat trying to mate with brother?

[debodun]

My two cats are 16 year old neutered males, brothers from the same litter. A few times recently, I've caught one going through the motions of copulation with the other. Why would he be doing this?

[opus88]

he is likely having some irritation involving his penis and needs a good urinalysis done, it could be crystals &/or a UTI (urinary tract infection) both need to be treated asap. male cats are especially prone to crystals especially as they age....always always feed male cats WET food, not dry....wet food is needed to keep their PH within normal range of under 7 and prevent crystals forming. please get your Vet to check for both these conditions

[debodun]

Thanks.