Section 21. If the guardian of a married ward that is incapacitated by reason of mental illness, is authorized under section eighteen to release an estate of homestead and the probate court finds that a portion of the proceeds of the real estate sold, or of an amount loaned on a mortgage thereof, must be reserved for the use of the ward, it shall order that a portion, equal to fifty per cent of the proceeds, to be set aside and paid over to such guardian to be invested in a homestead and held by the guardian for the benefit of the ward, if the ward survives the ward's spouse, the rent or use thereof to be received and enjoyed during the life of the ward by the ward's spouse, or until otherwise ordered by the court for cause; and the homestead to be said spouse's and to be conveyed to said spouse by said guardian, if said spouse survives the ward.