To be claimed as a dependent the person in question must qualify as either a Qualifying Child or Qualifying Relative. Obviously, we can rule out the child immediately.
There are four tests to be met to decide if either /or both is a qualifying relative.
-First, they must not be a qualifying child. This is met.
-Second, they must live with you for the entire tax
year. They do, so this is met. The fact that they are step-parents doubly qualifies them in test 3.
-Third, the person must have gross income under $3,300. This is all taxable income
they have for the year. If over $3,300 they do not qualify.
-Fourth, you must provide over half the support for this individual. This may be the tricky part if they are using Savings to help support themselves. The housing you provide will go a long way towards proving half of the support but if they buy their own food, take care of medical costs, and other expenses they may be paying more than half their own support.
If all four of those are met they will be a Qualifying Relative.
Now you'll need to pass three tests to take the actual dependency claim.
-First, they cannot be claimed as a dependent on anyone else's tax return.
-Second, they cannot file as married filing jointly
on their tax return (if they file a tax return).
-Third, they must be a US citizen.
If everything on those lists is met you can claim either, or both, as dependents.
You can read more on the topic in this IRS publication: