If someone is victimizing your wife's aunt, then you need to do everything you can to stop it. Not only is this man taking money from her that she might need later on, but he'll keep victimizing others if he isn't stopped. Talk with your lawyer or another knowledgeable person about your next step.
Your statement that your wife's aunt is getting forgetful also concerns me. Your letter doesn't say if she is living alone, but as her closest relatives you may need to examine her situation closely and see if the time has come for her to have better care. Her life could even be in danger if she's no longer able to cope with some of the things you take for granted, such as remembering to turn off the stove.
I hope you'll also be an encouragement to her. Let her know you love her, and you care what happens to her. Don't give her any reason to become suspicious of you or your motives (which can happen with someone who is having problems with mental confusion). The Bible tells us "to look after orphans and widows in their distress" (James 1:27).
Most of all, encourage her spiritually. Remind her that God loves her, and He will never let her go. Remind her, too, that Christ died for her, and someday soon she will go to be with Him.