He understands her partner found her unresponsive and alerted emergency services.
Police and ambulance confirmed they were called to a Gloucester St, Linwood, address around 2am.
Christchurch police have launched a probe but have said there is no indication that the death is suspicious.
"I found out half an hour ago, I can't believe it. I'm in shock,'' the relative told APNZ outside the house, which is wrapped in police tape, with officers guarding it.
"I was only with her yesterday afternoon, having a few nice, quiet drinks.
"She was on top form. She was so happy, she made me laugh all day.''
The man said his step-sister had been "in and out of hospital a lot lately with real bad asthma attacks''.
"She'd been pumped up on steroids, and had a back operation recently. That must be behind it.
"Police said the autopsy will be this afternoon and they'll let me know.''
He said the woman, a beneficiary, was a "cheerful, active person'' who did a lot of volunteer work in the community.
"If anyone was in trouble, she'd be the first to help them. She had a big heart.''
