It's rude to steal someone's recipe for their special sauce.
Maria is having her fiance's parents for dinner for the first time and wants to impress them with her cooking skills.
Maria wants to cook her sister's special sauce.
Maria tells her sister how important this dinner is for her and asks her sister if she would be okay with Maria cooking her special sauce.
Maria sneaks into her sister's room and takes her sister's recipe book without asking.
Maria's sister gladly accepts to give Maria the recipe and even offers her help with preparing the sauce.
Maria's sister finds out and gets mad at Mary for taking her recipe book and cooking her special sauce without permission.