She's actually the age of a grandmother, but she thinks that she is their mother.
So she gets angry when I call her a grandma.
ご質問ありがとうございます。
"She's actually the age of a grandmother,"＝「実は、彼女はおばあちゃんの年齢です、」
"but she thinks that she is their mother."＝「でも彼女は彼らのお母さんだと思っています。」
"So she gets angry when I call her a grandma."＝「なので、私がおばあちゃん、と呼ぶと彼女は怒ります。」
ご参考までに。
She is really a grandmother, but she wants to be a mother to her dogs.
She thinks she's the dogs' mother, but she's actually old enough to be their grandmother.
この場合、次のような言い方ができます。
ーShe is really a grandmother, but she wants to be a mother to her dogs.
「彼女は本当はおばあちゃんなのに、犬たちのお母さんになりたいんだ」
to want to be a mother to her dogs で「犬たちのお母さんになりたい」
ーShe thinks she's the dogs' mother, but she's actually old enough to be their grandmother.
「彼女は犬たちのお母さんだと思っているけど、実際はおばあちゃんの年齢なんだよね」
old enough to be one's grandmother で「おばあちゃんになるのに十分な歳」
ご参考まで！
アメリカ合衆国 
カナダ 
