More you have better it gets, 80+ plus she ain't going to be nimble any more but she still can enjoy some pleasure in her life till she passes. The population is getting older so why all the shock ?
Sound like you two are cute couple, her age wise is perfect for the late sex life. Mil's are great for affairs. Many (own) daughters (married) do allow it more than people think right under the family not knowing or do but just ignore it.