If by "real rule" you mean is it Federal Law, then no, it isn't. Is it part of the airline's own written safety policy? Probably. They don't publish those policies for passengers to see, so we have to take their word for it.
Either way, if the FA tells you not to do it, you cannot do it. These days ANY challenge to the instructions of an FA, no matter how polite, will get you an invitation to remove yourself from the plane. While you *could* ask her to show you the policy in her flight manual, that really isn't a practical option for the average passenger. It just is not worth it to argue with a FA, for any reason.
BTW, standard safety recommendation when one adult is travelling with two carseats in a 3-seat row is that the seat that has the longest front-to-back depth should go in the window, with the other seat in the middle and the adult on the aisle. The point is that no adult, including a parent, should have to crawl over a carseat in the event of an emergency evacuation. The only airline that I know of that consistently will allow a parent to sit between two seats is Delta. (The business about the protrusion of the seat into the pitch space is true, but most FA's prefer to err on the side of caution when it comes to barriers to evacuation. Again, insisting that they check the manual is not really practical, unless your goal is to tick them off.)