Is it stealing to go to your cousins party and bring food home. The family doesn’t mind if others take it and people take home food. I don’t know if they say it explicitly but that is the strong assumption.
This is something that depends on numerous factors, but largely it depends if your cousin genuinely doesn’t mind that food be taken home. This might depend on what you are taking, how much you are taking and your cousin’s personality and situation. If it is the norm to take home something small, such as a piece of cake etc., and there is enough for everyone at the party, then you can assume that your cousin will not mind.