According to Genesis 16:12 the descendants of Ishmael would live over against all his kinsmen. Who were these relatives?

First of all these were the descendants of his half-brother Isaac, the Israelites and the Edomites. Then there are the peoples who came from the sons of Ketura, of whom the Midianites are the most well-known. The descendants of Abraham’s nephew Lot, the Ammonites and Moabites, were closely related as well.

In the Bible many stories have been recorded about these peoples, who for centuries lived alongside each other.
It is remarkable how often Moab, Ammon and Edom and other nations were in conflict with Israel and how few conflicts have been recorded between Israel and the Ishmaelites. This indicates friendly relations between the descendants of the two half-brothers and is in stark contrast to the other nations in the Middle East.
For an overview of all the conflicts recorded in the Bible, go to  Study Conflicts with Nations