Could you hold a meeting and approve a change of locks? The front door of the apartment is common property and so is the front door of the block, so potentially you could approve a change of locks to just that one apartment and the front door of the block.
Then give him two weeks notice that the locksmith is coming, and new keys will be delivered to the owner, or to the tenant if the owner gives written permission for the tenant to receive new keys directly. The squatter-tenant will know that he’s not going to getting those new keys.
This gives the squatter time to move his belongings out. If he doesn’t move out, he’ll find himself unable to get in until his family member turns up and lets him get in and move his stuff out.
If you’re worried that he’ll cause damage during the two weeks after he receives the notice (because he’ll know he’s being targeted), make a motion that ALL locks are being changed “due to a recent security breach”. You don’t have to go through with that. Just change the locks that need changing and make a note in the minutes of a subsequent meeting that the security issue was resolved by changing only the locks of Apartment X and the front door.
The family member who owns the property should be prepared to help here – they are partly responsible for this as they shared their keys in the first place. They should be on site to supervise the exit of the squatter to ensure no damage occurs at that time (or they can hire security). And even if the squatter exits peacefully, you probably need to change the locks to prevent the squatter from returning with a replicated key.