Mothers Day in Brooklyn?
Mother in law is a traditional person but an adventurous eater. Would be just as comfortable with formal, old school white linen place as in a funky, trendy and hip . Want to book this now.
Bonus points for Cobble/ Carroll/ Red Hook/ Slope/ Ft Greene.
Convivium Osteria--very nice food in a unique atmosphere. Ask to be seated downstairs--either in the wine cellar or the enclosed patio.
Sounds like you don't have many constraints (which is a good thing). What sort of food does she (or YOU) like or not like? Consider Palo Santo, particularly if you can sit outside in the back garden - I've never been disappointed there. The roof at Alma has a great view and a meal to match. (I have an affinity for the outside.) On the more traditional end, Queen is a possibility. Anyplace that's popular here will probably fit what you've written above.
I don't know if any of the above places are doing anything special for mother's day.
187 Columbia St, Brooklyn, NY 11231
84 Court St, Brooklyn, NY 11201
652 Union St, Brooklyn, NY 11215
applewood, maybe? i really like their converted-to-a-restaurant aesthetic and have found the food to be really tasty (never been for brunch but the menu sure looks good)