grannyactivist, it must be an incredibly sad time as you strive to recover from health issues and grieve for your sister. I can only echo what others have said abut taking time (maybe whilst the funeral is happening) to sit and think about your sister and your memories of her, of being together, of shared times, secrets, and even the joyous moments. You must be proud of your son's achievements and I'm sure he would understand that you may not be in a celebratory mood right now. The highs & lows of life tend to be harder, more extreme, closer together, concurrent and consecutive as we age. I'd guess your sister would not want you to damage your health further by travelling a long way and would understand you not being able to attend a funeral service.
You have been the voice of reason, of support, of friendship and understanding to many on here (and no doubt in your life too) - know that there are people that will be sparing a few kind thoughts for you at this time. Also, I wish your son well and hope he continues to hit his goals in life.