How do I know if the same person manages multiple accounts on Twitter?

There are two accounts that tweet a lot of stuff equally, they say they are siblings, but they DM the same stuff and it is very weird.
I wouldn’t care if it wasn’t because the girl told my sister that her brother died, and trying to reach them we couldn’t. We searched the pictures on Google that the guy sent to my sis, but they are from differente people, even from politicians.
I was thinking that maybe the two accounts are held by the same person, playing terrible jokes, but they insist the guy died, yet wouldn’t tell where any type of funeral or event would be held.
Have any of you got any idea how to find out if the accounts are managed by the same person or on the same computer or something?