You've given your sibling what's called a gift of equity. @Anonymous_ has explained the tax implications for you.
Just as an FYI, there are also tax consequences for your sibling when he or she eventually sells the house. These are explained in this FAQ from the IRS website:
https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/property-basis-sale-of-home-etc/prope...
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
