Different colleges/programs use different definitions of first gen. Some consider you first gen if neither of your parents ever attended college, others consider you first gen if neither of your parents earned a college degree. None of them consider siblings, only parents.
First gen can sometimes be a tip factor for college admissions.
For college admissions, they don't actually ask directly, rather they ask for each parent: college attended (if any), degree earned and what year. From that information, THEY determine if you fit into their definition of first gen.
FYI: This is a FAQ here on CC and it is easy to use the Search function to find many pertinent threads.