Is it permissible for a non-Muslim to attend or assist in the washing of a deceased Muslim, especially if there is a familial relationship, such as the brother's wife being Christian?

It is permissible for a disbeliever to undertake the washing of a Muslim, provided that the Muslim intends the washing of the deceased and the disbeliever acts as his deputy. The jurists have stipulated this. It is preferable for the disbeliever to be present as an assistant to the Muslim who washes the deceased, such as handing him something and so forth. Al-Buhuti mentioned in Sharh Al-Iqna' that the apparent meaning of Ahmad's words is that the washing is not valid because the disbeliever is impure, while in Al-Furu' the validity was presented, and the washing was validated in the soundest of the two opinions.