I don't know how you can seem to expose yourself you either do it or not. But if you are taking her bra off then that in its itself is exposing. These cases are very rare but I know of at least one other case where a guys mother inlaw came on to him and they had sex not once but sevral times. You ask: What should I do. The right thing would be to not cheat on your wife. And if you care about your wife you won't do it. It is temping I know but just try to put it out of your mind. If you do do it you must be ready to the face the consequences if caught. If you are caught your wife may never trust you or her mother again and create bitter feelings that could last a lifetime.