In 1913 the 16th amendment was passed allowing federal taxation. Tax laws were organized into the internal revenue code in 1939. It was rewritten in 1954 and again in 1986. We now operate under the 1986 code as amended. Gifts are not income and they can be made to anybody.
Audits are random.
Flag system they come specifically after you. Computers pick up differences in each years return. If it is too far off the norm you get flagged. They generally can not go back more than three years from the latter of April 15th or when you filed. If you omitted income they can go back 6 years.