First, I would suggest running your scernario through the official IRS dependency tool.
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
Assuming you are providing more than 50% of the support for these individuals, and because of 'tie breaker rules" you are able claim your brother in laws (as well as your MIL) These rules state that when someone else can also claim them (their mother), but the mother doesn't (because she isn't required to file) you can claim them.
Even though these brother-in-laws may be under 17 years old, they would only qualify as 'other dependants" (the $500 credit) and not as qualifying children (the $2000 credit)
this topic is always complicated and because of all the different scenarios and rules, best to run through the IRS tool, like I did
