Dear Chang Yuan.
i know exactly how you feel, a form of detatchment that is unable to enter into the pain of the experience that your family are going through that makes you look like you dont care. i felt like this when my first grandfather died, but was too egotistical to care about what others felt at the time.
i guess that you can only be as supportive as you possibly can, helping out with funeral arrangements, paying some of the costs (or all). tell them that you are sorry that he has gone now, and remember his better qualities with compassion.
all you can do is love your wife. it was probably hard for you to have ever truly known your father in law because of his drinking. you need to be as supportive of her as you possibly can, because after all he was her father. be truthful and honest with her if she asks you why you are not in grief. falsehoods wont help.
have a heart to heart with your wife, if you get the chance. tell her that you feel guilty or uncomfortable that you do not share her grief. explain to her that it is because of your beliefs, and that you see hope for him in the future. i think honesty is the best policy. ask her if there is anything you can do for her. ask her how she feels about her father, is she angry, sorrowful, confused. what can you do. try to be sensitive, even if you dont share feelings of grief. go to the temple to pray for him in his 're-birth'. you dont have to tell your wife of these things. just do them.
love, white Lotus. xxx
if this advice seems inadequate, it is only because i have very little experience of loss in my own life.
i truly hope that this advice helps you Chang.