R..
I have a different outlook...you can hear me out or not...
I would have insisted on my brother going on his vacation once dad was through surgery and on his way to recovery.
WHY? Because you and your three other brothers were there to help each other out with the hospital visits and stays to watch over dad. I may not know hospital "mumbo-jumbo" but I can use Google and figure out what they are saying and what each medication does, etc. etc.
Him being there would not effect the outcome of dad's recovery one way or the other. He said his good-byes and maybe figured that being away when he passed would be easier than sitting waiting for him to die. Because being in the medical profession he KNOWS the probable outcome and knows the treatments dad is going to receive...doing it to someone else and not your own father are two different things.
He hasn't been around because he knows you guys are all mad at him. He probably wants to be there but fells like he can't be now. Offer up the peace pipe if you can...he knows he left you guys holding down the fort and he went off to have fun. Honestly probably not as much fun as you think...relationships with in laws can be very complicated...maybe even things at home with his wife.
I can say this because I am not super close to the situation and it isn't my dad or siblings (I only have one and her being there would actually make things worse.) Although if my family had a huge trip planned and something like this happened I would encourage them to go knowing I was there to keep them updated and keep watch at the hospital.
I am sorry he let you guys down but we all have to make our own choices and once he said he was going having everyone begging him to come back home was not right.
Life is short as you have discovered try and make peace with him...being stuck between your family and your wife/in-laws would be horrible. Good luck and I hope your dad makes a full recovery!!