How much can one body take?!?!?

My future sister-in-law just lost her 11 year old son to the same cancer that took her husband 6 years ago.  I don't know how she's surviving.  I know having the little one to take care of was how she survived losing her husband but how can you survive this on top of everything else???  She now has my brother (her future husband), her oldest son and lots of friends and family surrounding her but really how much can one body take!  You are never to bury your own children, it's not the natural order of things.  Bryan has suffered with this cancer since he was 2.  Looking for a bone marrow donor was how his father found out he had it too.  This little guy put up some fight.  For someone that wasn't supposed to survive to the age of 5, he fought the fight till he was 11.  He was an inspiration to my husband while he was going through his chemo and to anyone that knew him.  Randy would say "how can I complain, look at what Bryan's going through".  God Bless you Bryan, you're in good company!