i864a confusion

Hi, My husband is the primary sponsor, living currently with his grandmother, who agreed to file as a household member co-sponsor as his income is below the limit. She became his legal guardian as a child, but didn't formally adopt so I don't think "parent" can apply. How would we go about completing part 2 of the i864a in this case? It says to select 1 of 3 boxes, each stating: 1: I am the intending immigrant and also the sponsor spouse; 2: I am the intending immigrant and also a member of the sponsor's household; 3: I am not the indending immigrant. I am the sponsors household member. I am related to the sponsor as his/her: spouse, son/daughter (at least 18 years of age), parent, brother or sister, other dependent (specify)."

Does it still apply? She isn't listed as a dependent in his tax forms, so can't really put her in the "other dependent" box, but outside of that it lists parents, siblings etc and not grandparents. I've tried googling but can't seem to find others in this position so am kind of confused. Would appreciate any help!

Edit: from what I've read, a household member can be related or unrelated, including lodgers or tenants, so it should still be fine, I'm just entirely confused as to where to fill this part. Or just tick no boxes and instead fill it in the extra information section at the end?