Questions & Answers about Mi madre es realista: sabe que aprender un idioma lleva tiempo.
Because realista describes a more permanent quality or outlook, not a temporary state.
- ser + adjective is used for characteristics: es realista = she is realistic / a realist
- estar + adjective is usually for states or conditions: está cansada, está contenta
So here, the sentence is talking about the mother’s general way of thinking, which is why es is correct.