The pair had taken cocaine the night before and although emergency services attempted to resuscitate Miss Tomlinson, she was pronounced dead at the scene.
The inquest heard the former One Direction star made multiple attempts to help his sister get clean, often accompanying her to doctor's appointments.
Coroner Dr. Shirley Radcliffe, recording a misadventure verdict, said of the drugs: "It was a flawless storm, the oxycodone, the alpraxolam (Xanax) and cocaine".
Felicite - known to friends as Fizzy - was found unresponsive with "white lips" the morning after taking cocaine with a friend and medics were unable to save her.
It was a ideal storm, the OxyCodone, the alprazolam (Xanax) and cocaine.
Inner West London Coroner's Court heard the teen had a long history of drug use after the death of her mum Johannah Deakin, who died from leukemia in 2016.
But she left rehab early after getting drug free in November past year - once again getting in touch with friends who took drugs with her.
Recounting Miss Mohammed's version of events, Dr Radcliffe said: "A male friend came over and you did lines (of cocaine), one or two, but you felt unwell". "You saw [Felicite] do more lines but you didn't know how many she did".
In a statement, her GP Dr Paul Eulinger said the 18-year-old "refused to give up drugs in the knowledge they could kill her".
The inquest was told Miss Tomlinson was persuaded to complete a successful stint at an Egyptian rehab facility in Autumn 2018, but relapsed in 2019.
A tribute by her father Mark Tomlinson was read out at the inquest in which he described Felicite as a "a much loved daughter and sister".
Members of her family were in court to hear the verdict, but singer and X Factor judge Louis, 27, did not attend.
"She was an individual who took drugs deliberately and has succumbed to their said effects accidentally", Radcliffe said.