The advice is always 一起入境 or principal enters before the derivatives.
if 分开来美, the principal should enter first. The derivatives in theory are not supposed to enter before the principal, but when that happens in practice, they will be allowed into US almost always (although CBP can enforce the law and deny their admission). Then the catch is that the principal must enter before principal's immigrant visa expiration, or all derivatives GC should be revoked because they are now clearly issued due to errors.
if 分开来美, the principal should enter first. The derivatives in theory are not supposed to enter before the principal, but when that happens in practice, they will be allowed into US almost always (although CBP can enforce the law and deny their admission). Then the catch is that the principal must enter before principal's immigrant visa expiration, or all derivatives GC should be revoked because they are now clearly issued due to errors.
