Does a short haired cat with two short haired parents, that has a sibling with long hair that definitely had the same parent, carry the long hair gene?
If the parents are both shorthaired, but one of their kittens had long hair, we know that both parents carry the recessive gene for long hair.
When both parents are heterozygous (one longhair and one shorthair gene), each kitten has
50% chance of being heterozygous (one longhair and one shorthair gene)
25% chance of homozygous shorthair
25% chance of homozygous longhair
Each shorthaired kitten from those parents, therefore, has a 2/3 chance of carrying the longhair gene, and 1/3 chance of being homozygous shorthair and not carrying the longhair gene.
If you want to know for sure, you can test for the longhair gene for $40 through UC Davis.