you can't claim your brothers-in-law because they would be qualifying children to your mother in law.
the rules to calin a person as a qualifying relative are:
Qualifying Relative and eligible to be claimed as a dependent the rules are:
so it seems as best you can claim your mother-in-law if you meet the support test. you mentioned federal/state benefits so are they receiving something other than child support? you would have to check if claiming her would affect any actual or potential federal/state benefits. Those benefits for her if she actually gets them would count in figuring the support test.
