How do I convince mom for an assessment/ scan

Hi, (I'm from a 3rd world country and not proviliged). Mum's turning 55 this month. Has had hypothyroidism since childhood (her mom stopped meds for a few years when she was a child), was always impulsive, had emotional dysregulation and just wasn't very good at adulting. She used to have suspicions about dad cheating but we never took those seriously because they had a bad marriage and never had a reason. In the last 3-4 years her condition has worsened, from a few suspicons to being -

• Extremely religious, superstitious
• forgetting passwords and sometimes cash
• a few times suspicions about something being added to her tea, a camera, etc.
• sometimes she hears stuff, or recently she said she saw someone.
• is more irritable

Nobody took me seriously in the last few years, so when I could finally have a free doctors tele-consultation, both neurologist and psychiatrist told me it points to early onset dementia and needs a CT scan and assessment.

Now after this, finally my elder sibling who's working is taking me seriously but we don't know how to convince my mom for a checkup, as in what to say to her, how to talk about it? We don't have a very good relationship and I've been struggling w my own mental illness and more so because I'm unemployed.