Otherwise, it will be the last time you will get an honest answer. Noncompliance is socially undesirable, but remember that not taking medication is normal. Expect some amount of noncompliance. Ask about why medicines are being rejected without trying to argue the person out of his or her position. Your goal here is just to better understand what the reasons are, even if those reasons are not rational.
Persuasion is better than coercion.
It is better to try to find a way to persuade your relative to take medication. If you have read this far, I know that you have tried this tactic and failed. However, perseverance may work. You should continue to find a perspective on medication that both you and your relative can agree on. Perhaps there are some things covered below that you haven't already tried.