First you might do a bit of online research to find out the exact name of the order. There are various orders of Franciscans. You could also call the Diocese of Brooklyn and ask them what order taught at that school before closing. Contact the archivist's office.
Then find the order's headquarters. Most orders have some kind of archivist. Some answer right away, some not--like everyone else.
Explain that you want to include this relative who served God and the church as so often they are not included in family histories due to lack of detailed knowledge.
You might also do a first-name only search for "Angelus" on Ancestry for the relevant censuses. Most orders had only one person per name--unless they had a double name in religion, like Sister Miriam Regina, my great-aunt. Neither of those was her birth name. So your relative could be Brother Angelus Michael or whatever. Good luck.