I believe the sole issue with uric acid levels is that at excessive levels uric acid can crystallize in the joints causing arthritis-like pain known as gout. Uric acid is acquired by consuming foods with purines, which we metabolize into uric acid. (So 'which foods are high/low in uric acids' is a bit of a simplification and doesn't produce great search results).

Here is one site that lists amounts of purine and uric acid in various foods. Values for lentils seem pretty similar to meat products: https://dr-barbara-hendel.com/en/nutrition/tables/purine-content-table

However, I wouldn't advise that you focus on refuting your family's claims that lentils have so much uric acid that you should stop eating them. You may want to mention that lentils have comparable uric acid to meat. But I don't think your family is arguing in good faith. They may be cherry-picking anti-vegan arguments with little information or true concern about reality and are essentially just placing a huge burden of proof or justification on you for your diet (while they face no scrutiny).

So don't go after the hypothetical omni 'what if about nutritional unsoundness'. Say I'm glad you're concerned about my nutritional well-being. If you think I might be at risk for gout, let's go get me some bloodwork. And if my uric acid levels aren't anything to worry about, then you don't need to worry yourself any longer about my lentil consumption