If dad, mom and son (over 18) own a rental property (a house) jointly in USA, could the rental income be claimed by the son only?
Rental income is claimed by the person who receives that income.
If rental income is receives by son's only - it is claimed by the son. If however - that income is received by all co-owners - each must claim his/her part of rental income and expenses.
If the Canadian wife divorced with the husband (a US resident) in 2012 and comes back to Canada, they filed joint return in USA in 2011 and before, should the wife file US tax return in 2012 again if she doesn't have any income from USA except the alimony received from the husband who is still in USA?
Alimony is taxable income
in the US. The filing requirements are based on her total income.
However - please be aware that there is The US-Canada tax treaty - http://www.irs.gov/pub/irs-trty/canada.pdf - see page 24: 6. Alimony and other similar amounts (including child support payments) arising in a Contracting State and paid to a resident of the other Contracting State shall be taxable only in that other State, but the amount included in income for the purposes of taxation
in that other State shall not exceed the amount that would be included in income in the first-mentioned State if the recipient were a resident thereof.
So - if the wife is a resident of Canada and not US citizen - alimony is not subject of US income taxes and filing the tax return is not required.