A friend of mine faced this scenario today and told me about it. What would you do?
Her older daughter is 20 with a 5 year old son. They live at home with my friend and her other daughter, who is almost 18. 18 year old daughter frequently gets stuck watching her nephew, as the older daughter is a bit flakey and "forgets" about work, appointments or plans she made. This means that the 18 year old sometimes has to cancel her plans because she is the only one free to watch him.
Today, the 20 year old got called into work at the last minute. The 18 year old has had plans all week to go to an amusement park with her friends. She simply told her sister she wasn't doing it. Her sister was halfway out the door and said she would be fired if she didn't go in. 18 year old said she was just going to leave and would call someone saying 20 year old left her son at home alone, because she wasn't going to cancel her plans for the thousandth time.
The 20 year old ended up getting written up and is on strict probation because she had to stay home.
If you were their mom, what would you do? Would you punish the 18 year old at all?