"She had listed myself and my siblings as beneficiaries on the investment accounts."
In that case, the capital gains and dividend distributions made after your mother's death are income of the beneficiaries, not income of the estate, so this income does not go anywhere on Form 1041. If this income was reported on a Form 1099-DIV issued to your mother rather than to the beneficiaries, the preparer of your mother's final individual tax return (Form 1040) should forward the income to the beneficiaries using the nominee process and the beneficiaries will report the income on their own tax returns. (Because beneficiaries other than the estate were designated, the estate is not involved,)
