Just wanted to clarify in case anyone sees these statements and reads them as contradictory; what isn’t allowed is having sockpuppets, i.e. multiple accounts that interact with each other. Most commonly that would be IDing the other account’s observations, or third party observations that have already been IDed by the other account. In come cases commenting on the same content without being clear about being the same person could also be sockpuppetry.
Separate accounts that never interact can be ok. For example, I have seen cases where a person has an account that only posts or IDs observations of Taxon X, and another account that posts or IDs only observations that are not Taxon X. They state that clearly in their bio, and I haven’t seen any issue caused by it.
So I think it would be ok to create a separate account only for a grandparent’s pictures, but the relationship should be made that clear in the account’s bio, and the main account should never ID the grandparent account’s observations.