I'm not going to my sisters fifth birthday party.
Is there a reason for not attending your sister's party? Do you have other plans?
A party to attend with my boyfriend, which was planned the same day as my sisters. It's a good excuse not to go.
Children parties can be boring but they are important to your family members. They will feel hurt that you didn't go and it will damage your relationship with them.
I'm going to be with my boyfriend and it's way more fun whatever it is we are doing. He makes me happy.
Family is forever, and they might be hurt that you chose to go to a party with your boyfriend instead of being there for your sister. If your boyfriend wants your relationship to last, I'm sure he'll understand and you can make plans for later.