After their father was murdered, a gambler and his sister team up to take down a drug lord in their own hands in order for them to pay for their mother’s cancer treatment.

If the drug lord murdered the father there’s no mention of it here; it’s unrelated to the main focus of the story. You could just remove that part (plus “in their own hands” because what does that even mean) and it would make perfect sense, plus be stronger and more clear.

Don’t tell us more than we need to know, just give us one quick sentence that sums up the protagonist, antagonist, conflict, and stakes. You have all four elements and they are mostly clear, once the extraneous details are excised.

It also means something different to say “take down” vs “steal from” in regards to the drug dealer; dismantling someone else’s business doesn’t magically cover one’s medical bills.

“A gambler and his sister rip off the drug lord who murdered their father in order to pay for their mother’s cancer treatment.”