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.
'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.
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.