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In an interview with the Seven Network's Sunday Night program following Schapelle Corby's release, Mercedes says her sister is "broken now".
"I feel that now she really relies on me," she said in the interview, aired on Sunday.
"She has had to rely on me so she has lost her strength.
"She was such a strong person before.
"She is different."
There has been widespread speculation about how much would be paid for an interview with Schapelle Corby, who was convicted in 2005 of drug smuggling into Bali, but Seven has denied paying any money.
The programme captured Schapelle's first moments out of jail, including a sunset swim with her brother at a Bali beach.
Mercedes cried as she described acting as a nurse for her sister in Kerobokan prison.
"There were times I was allowed inside her room and I would bathe her," she said.
"For months we had to hand feed her.
"I would have to stick her medicine with my finger down her throat, hold a straw to her mouth so she could drink. She couldn't speak.
"They would have to carry her, she was like a zombie walking to and from her room."
Schapelle was not interviewed but some of her first words after her release were recorded.
After she was led from custody to a waiting black van and through a bustling crowd of media last month, Schapelle said she felt used.
"I started crying because I just feel so used," she says with her face covered with a scarf.
"They just use me. People judge me and say I am a really bad person but look at this.
"I don't like to judge but that's wrong," she said, as the photographers swarmed all over her van.
Mercedes also dismissed rumours her late father Michael Corby was linked to the four kilograms of marijuana in Schapelle's body board bag.
"All those rumours that have been said about my dad are wrong," she told Sunday Night.
"They are untrue and if my dad was alive now, they would have never been published.
"They would have never been said because he could defend himself."
Mercedes said she wanted the marijuana tested, footage from Australian and Indonesian airports and for fingerprinting but got nothing.
She also reiterated a defence used in the case, alleging airline baggage handlers were involved in planting the drugs in Schapelle's bag.
"I don't know who put it in that bag but I am pretty sure something to do with someone who worked in the airports," Mercedes said.
Indonesian authorities have indicated they will examine the program.
Authorities have warned Schapelle against participating in an interview, which could land her back in jail for breaching parole.