我有一个妹妹。

Wǒ yǒu yí gè mèimei.

I have a little sister.

我明天和妹妹一起去看电影。

Wǒ míngtiān hé mèimei yìqǐ qù kàn diànyǐng.

I'm going to watch a movie with my little sister tomorrow.

我妹妹已经上大学了。

Wǒ mèimei yǐjīng shàng dàxuéle.

My little sister is already in college.