Nice wife. i think you are hurting her, by rubbing her sphincter with your fist for long minutes. that is a hard way to stretch. Get an inflatable butt plug, Put it in while it is small, then pump it up so it becomes large inside. then she needs to push it out enlarged. Hurts much less. It also helps to get a piece of a water hose, one inch diameter, and blow some air into her ass with your lungs, without the hose actually entering her. just lean it on, on the hole.. That way her asshole will get a shape of a balloon inside. She should keep the air from escaping out, and than you can easily put your hand inside in a quick one movement. Both of you will like it. try it out.