Any topic can be a good one if it’s handled skillfully but this one may be a bit tougher to turn into a diamond. There are a lot of middle children applying to Rice. It’s going to be hard to argue that your middle child experience is uniquely different from everyone else’s middle child experience, and given you qualities that the other middle children don’t also have. It’s possible. But difficult.
You might do better to pick something else that is more unique. Surely you have some aspect of your life that isn’t common to 50% of the applicant pool.