I've heard rumors my mother has had other children besides me, what website can help me find records to either prove or disprove these rumors. (xpost r/genealogy)

I always thought I was my mom's only child up until two years ago. My uncle told me a rumor that she had three other children but put them up for adoption. Apparently this was some big family secret to protect me. I'd like to know the truth, but I don't want to start a lot of drama. What websites can help me find records on what children she has? If it helps they were closed adoptions.