Final answer:
Lisa can only claim her current husband as a dependent since he has no income and is not claimed by anyone else, meeting the IRS criteria. Her stepson is not a dependent due to his age and income, and her ex-husband and former brother-in-law cannot be claimed due to residency and relationship limitations.
Step-by-step explanation:
Lisa may claim one dependent, her current husband who has no income and is not claimed by anyone else as a dependent.
In order for someone to be claimed as a dependent, several IRS tests must be met including relationship, gross income, and support among others. Lisa's current husband qualifies as a dependent because he has no income and she provides more than 50% of his support. Her stepson, although she provides more than half of his support, cannot be claimed as he is 19 and not a full-time student, plus he earned more than the allowable gross income limit for dependents for that year. Lisa cannot claim her ex-husband because he does not live with her and they are not related by blood. Similarly, Lisa cannot claim her former brother-in-law as he does not live with her, they are not related by blood, and there is no special condition that would allow such a claim post-divorce. Therefore, the number of dependents Lisa may claim on her tax return is limited to her current husband only.