What risks could I face if I open a U.S. bank account on behalf of my aunt and uncle?

Very simple. You'd be responsible for any and all activity that happens in your account. I don't think the defense that your Uncle made you open the account would hold up.

You're of legal age, you opened the account. You signed the dotted line. You gave your Uncle/Aunt the credentials to use the account. You're liable.

Allowing someone else to operate your own checking account is also against the banks' TOS.