Elder dependent in-law

Can a son-in-law (tax payer) claim his mother-in-law as dependent to get transfer of her DTC? I tried adding her to his return but the field for relationship has only “mother”, it won’t accept mother in law. The daughter (tax payer’s spouse) does not have any income so can’t use the transfer. She does not live with him but does financially support his mother-in-law.

A dependant may be:

Your parent, grandparent, child, grandchild, brother, sister, aunt, uncle, niece, or nephew
OR
Your spouse’s or common-law partner’s parent, grandparent, child, grandchild, brother, sister, aunt, uncle, niece, or nephew.

Yes but the disabled person needs to have relied on that support person for
necessities of life (food, shelter, clothing). CRA needs to approve the support person first. It’s on the first page of the T2201 but if you want to add or change a support person after the DTC has been approved, the disabled person needs to send CRA a letter.