I don't know where you live. If you live close enough to a town that has a restaurant names Midevil Times, we took three of our kids there for there for their 16th, 18th, and 21st birthdays. There are actors who fight with swords on horses just like in midevil times. It was an experience we will never forget. However, if you don't live close to one of these restaurants you may want to consider something that both of you enjoy, i.e. Four wheeling or something else.
Where Could I Go To Celebrate My Birthday Party I'm Going To Be 12 And It's Joint Party With My Brother Who Will Be 15 It Needs To Be Nice And Maybe A Place To Eat Any Ideas ?
How about renting a motel or hotel room where they have a pool, indoor if the weather is bad. The folks could also rent one that way girls and boys can change in separate rooms and parents are close enough to chaperone. I did this for my son, and we all had a blast. My niece and nephew were younger than my son, everybody had a good time.
Y don't you got to a motel and a pizza hut , make sure the motel has a pool