Odd eligibility stimulus check question

[lendra-zain-tsun]

I have an older mentally challenged family member. They arent old enough to draw scocial security,  but they are comming up on it in a couple years. They have not held a job in quite a few years. We all live with our grandfather who draws scocial security but does not file for taxes. So technically my family member in question is a dependent of him, but not on paperwork. Would they be elidgeable to get a stimulus check, and where might i find the resorces to sign them up for it?