Chaumian mints such as cashu can provide custody without the custodian knowing which transactions or which coins belong to whom.

You could have a mint for only your family members. You admin the mint with the BTC unter multi signature control together with your three brothers and then suddenly you see the balance go up by 12 BTC but you would have no way of knowing who just deposited this and received an equal amount of tokens. You could abuse these funds and then the respective relative might step forward and demand you honor the promise but else you would have no way of knowing on who's behalf the mint was sending out BTC or LN payments once the tokens get redeemed.