Am i wrong

My father refuses to hold my 19 wk old son whom weighs 15lb 10oz. Hes a chubby baby, a big boy. He sits really nicely hes a real snuggler snuggles right into you and doesn't fidget much. My father will hold  my sisters 2 wk old baby whom weighs 6lb because shes different he had no choice the nurse gave her him and hes grown attached to her at home! I put my son on his lap literally today and his reaction was ' get this kid off me now' !! I was upset but i refuse to argue infront of children so calmly and quietly i got my stuff together and said i need to go to pick up my girl from school.  Luckily it was school time nearly to get away. Im pretty angry and upset. My sister told me im over reacting being selfish and a spoilt brat!! I can't understand how you can accept one child to hold but not another especially when their both your gran children. Am i wrong?? I am feeling so upset tonight which isn't helping my post natal depression. Thanks for replies/votes