When the time comes make sure you take what is yours, it is what your father wants.
I caused a family rift when I did exactly the same thing. My grandmother insisted we split her estate between myself and 2 siblings. I have 3 siblings. After grandmother died my siblings said we should split the estate 4 ways. I said the only way I would split it four ways was if we put one quarter in a legal trust for the omitted siblings children (2 girls). I stated the money was immaterial, it was the wishes of my grandmother I was upholding - and by giving the money to her children I was not disobeyed her wishes. My sister said "no way, I want the money", so I took the full 3rd and haven't spoken to her since
My grandmother raised me for several years when I was aged 2 to 5. It was important to me that I did this one thing for her. She was adamant my sister should not get anything (my sister is a money grabbing, using, useless waste of space with a deadbeat loser of a husband)!
Take those guns, and keep your father proud. What if the brother-in-law pawns it, loses it etc - you'll never forgive yourself.