Caring for his sister is a major responsibility and he should consider it like a job for his application. It should easily explain his lack of ECs.
In today’s world of college admissions, nothing is guaranteed. But he has done very very well so take a deep breath and cast a wide net for schools – reaches, matches, safetys. IMO as long as there is no focus on one hyper-competitive school or group of schools people end up just fine.