My kittens opened their eyes on the 10th day after birth so do they have long fur or short fur? I am asking this question because I heard from my friend that if kittens open their eyes between 5-8 they will have short fur and if they open their eyes between 10 to 14 they will have long fur. The mother cat is a Birman cat and I am not sure about their father cat because the one who owned her before me had abandoned her and I found her injured and already pregnant.
That is not a reliable method of determining the length of a cat's hair coat. Time will tell how long your kittens' fur will become!
Cat hair length is determined by both the mother and the father cat, but there is no steadfast rule. For instance, in a litter of kittens with one short-haired parent and one long-haired parent, some of the kittens could be short-haired while others long-haired and still others medium-haired.
Very similar to eye color in human babies, hair length in cats can also change significantly in the early months of life. As kittens grow into their adult fur coats, even seemingly short or medium-haired cats can grow a longer adult coat. Some kittens will have longer hair early in life, but as they grow into their adult coats, their hair becomes seemingly shorter.
Unfortunately, what you were told was nothing but an "old wives tale". There is no steadfast rule to a cat's hair length except to see what time can bring!
All the best,