The phrase "son of a sister-in-law" is a mouthful, and it's not always the most convenient way to refer to someone. Luckily, the English language offers a plethora of synonyms to substitute for this complex phrase. Some alternatives to "son of a sister-in-law" include nephew, cousin, or even just in-law. Depending on the context, you could use more specific terms like "my spouse's sister's son" or "my brother's wife's nephew". Ultimately, it's important to consider your audience and the situation when choosing which synonym to use. No matter what word you choose, clarity and kindness in communication are always key.