atilla:
Brotherly. Try and see your wife's brothers and sisters as your own. I know u def will treat your own harsher e.g if you come home and meet your brothers and sisters at your house without telling you, you probably would send them back home or wake your sleeping brother with a knock.
But don't say anything when the food finishes then let everyone go hungry and they will know not to come next year. Just as long as your baby is okay let everyone else enjoy the decision they took without you.
What do you do for a living. Maybe you can take the 20 year old with you for a day in your office so they understand that you work for your money and its not easy. Just try and be gentle cause your wife will understand later but for now she won't understand and will get emotional about it. Another thing talk to the dad yourself he should understand

T r a s h