IMO: since they are your clients (whether you are paid or not), I would suggest individual accounts for all, or the parents could have their own family(business) account together.
From there, shared vaults would give people access to the passwords they need. One for you, one for your girlfriend, and a shared one for the two of you. The parents could do the same, and also create a shared vault for the business.
This gives you good granularity about who has access to what. If the business is sold, or any of the relationships go sideways, etc. fall out should be minimal.