Kate's mother was born working class/lower middle class. She married well in a social sense and is a successful business woman in her own right. She is now upper middle class, albeit slightly nouveau riche.
Kate's father was born upper middle class-ish, with trust funds for education but very little hard cash. He took a professional job, and joined his wife's business venture when it started becoming successful. He married well from a financial point of view. He is now firmly upper middle class.
As a child, Kate was a professional middle class heading towards upper middle class. Her expensive education was paid for by her father's family's trust funds and her parents became multimillionaires when she was a teenager. At least since leaving school if not before, she's been firmly upper middle class.
The Middletons also appear to be a close knit family that get on well with each other. Carole and Michael appear to have married well in the sense that they have been married for 40+ years and still seem happy together (ie, in the best sense).
Interestingly, Kate Middleton, Sophie Rhys-Jones and Timothy Laurence all have surprisingly similar backgrounds although Kate's family started with a lot less money and now have a lot more money, compared to the other two.
William has an upper class family background, with his mother's family being more established than his father's, if such things are important.
The Windsors' issues are well known.
William might have cheated on Kate, but there's no definitive evidence either way. There are widely reported rumours in relation to a specific affair which you can choose to believe or disbelieve (or not care about), and that's it.