Paz Orquiza was travelling from Los Angeles back to Melbourne, Australia, after visiting relatives.
Her granddaughter Marianne Santos Aguilar said Mrs Orquiza has a number of disabilities including severe arthritis.
Her relatives had all chipped in to buy her a Business class seat with United in the hope of making the 16-hour journey more comfortable.
But Ms Aguilar claims her grandma was left "in tears" due to her treatment by United staff.
She claimed staff would not allow Mrs Orquiza's daughter Rose, who was also on the flight in Economy class, to help her with tasks such as reclining her seat or opening her food unless she paid for a Business Class ticket as well.
Neither would staff themselves help the fragile passenger, the family claim.
They claim they were left with no choice but to move Mrs Orquiza out of Business and into Economy so her daughter could assist her, despite having paid for extra comfort.
According to Ms Aguilar: "My aunt tried to comfort my grandmother, who was in tears, and was horrified to see how much pain she had to endure during this flight.
"Upon arrival to Australia, my grandma's legs had swollen, she suffered from a stiff neck and her whole body ached.
"She is still in great distress and pain from this ordeal."
The family have filed a complaint with United and say they were eventually offered £400 in travel vouchers and a refund of £690.
But the family remain unhappy..