Hello,
For relatives living in Brussels here are few possibilities because of the privacy law, as they maintain a 120year privacy which means you can't really get any information after 1900; Unless  you go there personallly and can prove that you are a relative of the person or persons you are looking for, which ofcourse is difficult for 2 reasons, I presume you don't live in Belgium , and because of the Covid-19 problems now.
Hopefully you can find a way to find your family.
Regards,
S Stein