Are there any other family owned valuables that your uncle has that might be better removed from his care at the moment?
Is it worth investing in a safe for yourself? You mentioned that your Dad would be furious if he found out about the ring. Getting your Dad to look after it would involve either telling him what has happened or lying to protect him from the truth. Or would it just be "Oh, now you have a safe you can keep it yourself".?
I'm also slightly evil and make sure the daughter (and her siblings) know exactly where their father got the funds from. That way they will hopefully not expect such extravagant gifts for their birthdays and hopefully won't guilt him when/if they don't receive them.