How serious is this? If this is new, and she is having serious mental lapses, then that could be a serious medical problem.
If she is just scatterbrained, then she is like lots of other people. I assume you knew it when you married her.
Lots of people have quirks, habits. I still get my hair cut the same place I did 20 years ago. It works, so I don't look for anywhere new. Maybe that is how she feels about oil changes.
If her sister is from a different marriage, the why is 12 a stupid answer? If not, then it was a stupid question, and didn't deserve a straight answer.
You are her husband, her partner. You are supposed to have her back, to cover for her, to be strong on things where she is weak, to protect her, help her, love her. She should do the same for you. Attacking her for her weaknesses is no doing that.
My wife has no common sense and it makes me crazy. She does and says stupid things and she does it so much to the point that I am constantly worried about her well being. She is just constantly doing and saying things that makes no sense at all. For example, she has two older sisters, ones a half and the other is full blood and they are all four years apart from each other. A friend asked her how old was your oldiest sister when your mother met your father. My wife says "Uh... I don't know like 12." I got so embaressed and she does stuff like that all the time. I don't want to leave her cause I love her but when she drives to another town to get her oil changed instead of looking for one near by it just drives me crazy and it keeps me in a bad mood. I have told her on plenty of occasions to think before she does or says things but she just gets mad at me and yells about how I think shes stupid. I feel bad because she is the sweetiest person in the world but I can't handle her not useing her head anymore. What should I do? I don't want her to get herself killed cause she does not think before she does things and believe me it came close a couple times.