This is a false generalization; for proof of which I submit the following:
My wife's father had a family heirloom, a Winchester Model 1894 made when McKinley was president. He had received it following the death of his older brother who had been a sheep rancher during the 1920s & 30s in Oregon. the rifle was used while herding the sheep in combination with the good sheep dog to ward-off or eliminate coyotes and possibly a remaining wolf. My wife has a brother but he has little interest in firearms, and recognizing that I would give it the respect it deserved, her father gave it to me. Over 40 years later I still have it. I obtained dies and moulds and load it with low pressure lead loads which cause little wear on the rifle. My brother-in-law has even enjoyed visits where we have taken it out to shoot. There is no jealousy on his part as he is glad to have me provide the care for the firearm that it deserves.
Exhibit 2.
On nearing his death of cancer my father divided his gun collection between my brother and me. My brother was older and as such was always dad's favorite. As such the really nice guns went to my brother. Some are irreplaceable such as an early production Browning superposed with Twin triggers (not double triggers), a Spencer .56 carbine from the Civil War and a few other really collectable items. One of my brother's personality characteristics is the need for instant gratification and he is willing to get rid of any future value for something now. this has led to him divesting virtually all of dad's guns. He did this without any notification to me or any offer for me to purchase them from him. Some of them were sold so cheaply that I would have gladly doubled the price that he got. But since he always resented me (he was dad's favorite but I was mom's favorite), he felt justified in preventing me from getting anything I valued.
So there you have two situations with an in-law or blood relative. Too bad the fathers didn't recognize the values of their decendents and taken steps to have a better result.