No body wants to see there mother and father separate, I am no Dr. But it sounds like he(your dad) might have some problems after loosing his dad. The main thing is is to hang in there. Did you ask him about walking you down the isle? Sometimes people needs space after a loss of a family member, I have seen people that love each other for thirty plus and wake up one morning and say they do not love each other anymore. It is very difficult, it is not yor fault or your brother or sisters fault or your mom's fault, it is something that happens beyond our control!
Your dad still loves you, my dad loved me but he was the type that never expressed his feelings, just like his dad. Your dad just needs time and it is his loss, not yours, he is the one that is missing out on his family all we can do is hope that he can come to grips with his problems.
My thoughts are with you and your family, call him and ask him about visiting him and try to have a heart to heart talk with him!