@Emanuele124
If the account still shows you as "owner" then Airbnb considers you the owner, no matter how the payouts are split. There is no "assumption" per se.
The proper way to have setup the account would have been for the account to be correctly setup with the relative as owner (since they are the actual owner) and with you as Co-Host. You could then become "Primary Host" from the guest side, but the actual owner on the profile is the property owner - not you. You could then assist them as a Co Host as needed.
Unfortunately, there is no way to "fix" this. Airbnb does not allow you to change this, or transfer ownership of a listing. You could setup a new listing, but all the reviews will stay with your profile, because reviews stay with the "owner" of the account. Your relative would have to start over. Additionally, Airbnb has been suspending "duplicate" listings, so if you were to create a new listing, it could be suspended as a duplicate.
Primary Hosts vs Co Host vs "Owner"
https://www.airbnb.com/help/article/1536#section-heading-2-0