Tax liabilities on money that you give to your sister each month will really depend on how that money is being classified.
In other words, if what you are paying her is basically a rental fee, then according to IRS regulations she would need to report that as rental income.
However, it sounds like the money you are giving your sister amounts to more than just rental fees, and also applies to groceries, home repairs, vacations, etc.
Money that you would give your sister to help pay for groceries or utility bills is not taxable income to her, as all you are really doing in that case is reimbursing her for your share of groceries that were purchased, and your share of utilities that were used, etc. However, an acutal rent payment would be income.
Under current IRS regulations, you may give a gift of $12,000 annually to another individual with no gift tax reporting due. That being the case, it might be a good idea just to classify $1,000 a month to your sister as a gift, and then the remaining money that you give her as reimbursement for your share of groceries and other living expenses. That would eliminate the need for your sister to report any rental income.
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