Is it sufficient to give away stolen valuable items to charity, without informing the relatives from whom they were stolen and asking for their absolution, in order to remove the sin of theft, given the impossibility of informing them for fear of severing ties?

Theft is a major sin, and repentance from it involves returning the stolen item to its owner or seeking their forgiveness. Donating it as charity is not sufficient unless the owner cannot be identified.

Regarding seeking general forgiveness without the knowledge of the right-holder, scholars have differed. The most prudent course for your faith is that the right-holder be aware of the right from which they are forgiving you.

When returning the right, it is not a condition to inform about its nature. Rather, it suffices to deliver it to its owners through methods that suit you and cover your matter, without exposing your situation.