Is this a joke??? What would you do?

Me and my boyfriend have not long moved in together so everything is new etc, I miss being at home and sometimes when my boyfriend is out have my little sisters around for tea or for an afternoon (who are 11 & 5 years old) I am 23. Now my boyfriend has younger brothers 21years & seventeen almost 18 years old, and it seems like because i have my sisters around his mum & dad like us to babysit his 17year old brother when they are away on a night or something!! So he has to come around, I have to cook tea (as his mum buys the food for me to cook for him!) and he sleeps over. Is this a joke, bear in mind he has A girlfriend and is going uni next year, oh and he went to the leeds festival for five days in the summer but we have to babysit for a night, a friday night! I dont even have a say in this either. What would you do?