If you are citing this as an example from a piece of writing, I can help analyze its literary use, or help rephrase it for clarity if the original intent was different (e.g., “knocking on my brother’s wife’s door”).
I notice the phrasing you've quoted could be interpreted in multiple ways, including a possible typo or an intentionally shocking phrase for literary effect. However, the phrase “knocking up” typically means “making pregnant” in informal English, which would describe a non-consensual or taboo situation. Knocking up my brother-s wife- My sister-in-law...
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