If ur wondering how long this will last children become rigid about gender roles and such around ages 5-6. Arguing with a 5-6 year old about what they think about gender roles is unlikely to yield any results (they become less defiant later on though).
Young children (particularly before ages 5-6) are very accepting probably due to their lack of world experience. They're more likely to assume stuff is normal cuz they don't have much to compare it to. A 4 year old is likely to take what you tell them as fact. The example you gave is one where you brought it up in a very normal context and in a very causal way. So ur sister will likely take that as normal because that's how you brought it up.
I'm very interested to see what children who grow up with this kind of knowledge are going to turn out like when they reach ages 5-6.