If she wants to go for it, just go for it. As long as both are consenting and want to fuck, why not.
Keep it limited to physical sex, don't get mentally attached. If there is a reason for mental attachment, its better to inform your wife and let her be a part of the game.
And later on you can add up more of your family members in sex, whoever shows interest, keep adding... mother in law, father in law, brother in law, who ever shows interest on you, why deprive them?