Hello,
I had the same situation. Spoke to cic contact center agent and then also queried the Twitter account of IRCC. Both said the same thing - family size of both the application that is your and your wife would be same . So in your case it would be 8 on both application.
And in the financial evaluation form, on your application your in-laws would count under previous undertaking not in effect signed as co-signor and in your wife application financial evaluation form, your parents would be counted under previous undertaking that spouse signed as co-signor.
Also, I included a letter explaining this full thing so as to keep the processing officer on the same page. In that explain letter , I referenced the other applications confirmation number