追你姐当你姐夫
[zhuī nĭ jiĕ dāng nĭ jiĕ fū]
It roughly translates into 'marrying his sister in order to become brother-in-law'. Generally, this kind of idiomatically expressed personal information is meant to create an unexpected relationship dynamic. But this nickname does carry the cultural significance of showing admiration or pursuing a woman older in terms of her kinship ties, rather than directly romantic interest.