Hi Jan,
This has only come up in the past for me for special events. For example, when we hold a big fundraising dinner or even the volunteer banquet, sometimes a family relation will pitch in. The volunteer is usually only there for the night. Since I haven't had a problem in the past, we have allowed this. If both individuals are adults (i.e. the volunteer is not a teen), I think it is okay for them to be related to their supervisor, as long as the position has a certain distance from internal workings. Such as, I have had volunteers help with filing in the accounting department, but I don't think it would be wise if they were related to the accountant!
I think if the volunteer is planning to work long-term, I think it would be helpful if they report to two people. They should have an "activity" supervisor who oversees/helps with their actual volunteer activity, and also the volunteer supervisor/administrator. One of the staff is the relation, and one of the staff is a third party just in case. This is obviously easier in a museum with at least five or so staff.
In terms of evaluating and helping this volunteer make changes to the performance of their duties, I think this is where peer evaluation comes in handy. Volunteers who are trained to help each other can make observations and suggestions about performance that a family member might not. The webinar AAMV is offering on January 25th (see the "News" tab) might help with this.
Hope this was helpful! Please feel free to report what you decide and how it works out on the forum. Also, if anyone hasn't done so already, you must hit "subscribe" on the forum's home page to receive email alerts about new messages.
Samantha Roberts
Events, Membership, and Visitor Services Coordinator
Abroms-Engel Institute for the Visual Arts