I would like to pay an expiation on behalf of my sister, without her knowledge, for leaving one of Hajj’s obligations. Is this permissible?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

It is valid under the Sharee'ah to pay an expiation on behalf of your sister from your own property. But taking her permission is necessary as long as she is alive because this is an act of worship, which needs an intention from the one upon whom it is due. That person should intend to do it by himself or give permission to another to do it on his behalf, according to the words of Ibn Qudaamah in Al-Mughni.

Allaah Knows best.