She said that she made sure the 27-year-old 'Rehab' singer, who was found dead at her Camden home in July, looked "beautiful" in her favourite yellow dress and trademark beehive wig before her funeral took place.
Janis told the Sunday Mirror: "It was an act of love. She went out looking like the superstar the whole world fell in love with.
"When I looked at Amy's body, that it dawned on me she'd gone. She looked peaceful, at rest, beautiful. It was my moment of final acceptance. The time to say goodbye. It was Amy's curtain call. As a family we'd decided Amy's funeral should be a party - a celebration of her amazing life."
Janis said that she was "stunned" and in "denial" when she first found out about Amy's death, saying: "It wasn't real. That is why I had to go and see Amy."
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She also revealed that she was "carried away on a wave of emotion" with the "warmth" from friends and family and found it "very touching" that the singer's fans camped out outside her home. "I realised I was not grieving alone," she said. "It was comforting."
Janis said that she finds it "too painful to hear Amy's music", adding: "There've been lots of tears. The bad days are very bad. There are bits of time when it just gets me. Often it's at night. It will hit me... this is it, this is reality."
The 55-year-old insisted that she did "absolutely everything" she could to help Amy overcome her drug addiction and still believes that she was drug-free "from November 2008 to the day she died".
"Up until then she'd been a nightmare," Janis said. "But Amy was working really hard to turn her life around. Part of me feels comfort that she's no longer in pain."
Watch Amy Winehouse and Tony Bennett's 'Body & Soul' video, released to mark the launch of her Foundation, below: