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What with all her aunts, uncles and cousins, she has many relatives.

What is the meaning of the above sentence?

My guess is to write 'what with' either BrE/AmE word. It could be a slang or infomal word too.

The following is simple and everybody understans.

1.She has many relatives as she has aunts, uncles and cousins.
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'What with' means 'considering' or 'on the evidence of'. I use the phrase, but I would hate to try to parse it; its currency across the Water is unknown to me.
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I would not describe "what with" as slang. It's informal for "in view of" or "because of". It's American, but I would not be surprised if it were also British.

What with the heat and humidity, we suffered all week.

CJ