The 'Mysterious Girl' singer returned home from his native Australia today (February 25) after rushing over to see his 52-year-old sibling Andrew.
"Pete has been knocked for six by this," a source told The Sun. "He found out last week that Andrew was ill and that it was actually really serious.
"He was in Africa but once he got back to England he flew straight out to Oz to be with his family... It's a real nightmare for everyone.
"Andrew is seriously ill and while the doctors have acted quickly by locating the cancer and making plans to operate, nothing is being taken for granted. It's a very tense time indeed."
Taking to Twitter this afternoon for the first time in 15 days, Andre told his followers: "Hi guys, back from Australia now , a very tough week... Spending the weekend with [children] Harvey , Junior an pringles and I am smiling . Thanks so much for your messages."
Andre suffered a health scare of his own in 2007 when he was diagnosed with meningitis.
> Peter Andre: 'I may quit showbusiness'