Unless the trust instrument provides otherwise:
(1) Termination of the interest of the sole income beneficiary prior to the termination of the trust causes each principal beneficiary to become a beneficiary of income in an amount proportionate to his interest in the principal.
(2)(a) Termination of an interest in income of one of several income beneficiaries causes the other income beneficiaries or their successors to become beneficiaries of that interest in income in proportion to their interests in the balance of trust income.
(b) If, however, termination of the income interest is by death, and if descendants of the deceased income beneficiary are the beneficiaries of an interest in trust principal or succeed to such an interest upon the death of the income beneficiary, such descendants shall become beneficiaries of the deceased beneficiary's interest in trust income in proportion to the descendants' interests in their portion of trust principal.