Two plants look alike — both have thin, green, grass-like leaves and both grow in clumps

Why

Tanya has done the first, natural step — noticing a shared feature (grass-like leaves, clumped growth) — but she has stopped at the most visible one. Plants are grouped by looking at SEVERAL features together: the leaves, but also the flowers, the roots, the seeds, and the stem. Very different plants can share one surface feature: onion, garlic, wheat, and lemongrass all have thin blade-like leaves and grow in clumps, yet they are quite different plants with different flowers, roots (a bulb, versus fine fibrous roots), and uses. Test it: wait for each plant to flower, or dig gently to see the roots — that is usually where two 'grassy-looking' plants turn out to be nothing alike. Her brother is right: a single shared feature is a starting clue, not a conclusion — grouping needs the flowers, roots, and seeds checked too, not just first impressions.