Israelites and Midianites
Why are the Midianites enslaving the children of Israel around Gideon's time? Aren't they the same Midianites into whose family or race Moses married (Jethro, the Priest of Midian)? How did they become enemies? Or are they a different Midianite group?
The questioner is correct in thinking this might be the same group. The Israelites and Midianites go way back -- to the time of Abraham. Midian was the fourth son of Keturah, Abraham's third wife. This son, along with all the other sons of his concubines, was sent "eastward" while Abraham was still living so Abraham could give all that he had to Isaac (Gen. 25:1-6).
The next mention of Midianites occurs in a history of kings in Gen. 36:35. There is a side mention that Hadad, the son of Bedad, smote Midian in the field of Moab. Next, they appear in the Joseph story. Some texts have both Ishmaelites and Midianites. Obviously, they were merchantmen, traders (possibly slavetraders?).
When Moses fled from Egypt, he went to Midian. There he met Jethro and married his daughter. There he received his commission to lead the Israelites out of bondage. It is even possible that Mount Horeb was in Midian. Some scholars argue that Yahweh (God) was already known to the Midianites because Jethro was a priest who not only gave sage advice to Moses, but also presided over a sacrifice and sacred meal in the presence of Yahweh. Others think that Jethro became acquainted with Yahweh through Moses. Either way it suggests some connection between the Midianites and Israelites.
But it didn't last long. Before the end of the wilderness wanderings, the Midianites teamed up with the people of Moab to expel the Israelites from the land. They were part of the whole fiasco with Balak and Balaam, where Balaam was hired to "curse" the Israelites, thereby preventing them from settling in the land (Num. 22:4ff). Although that plan didn't work, the Israelites soon adopted the idolatrous practices of their surroundings, culminating in the marriage of an Israelite man and Midianite woman. That brought down severe repercussions in the form of a war of vengeance in which thousands of Midianites were killed, including women and children (see Num 31:2ff).
Somehow, though, the nation recovered. And when Israel again disobeyed Yahweh, he delivered them into the hands of Midian for seven years. At this point the Midianites were very powerful. It is thought that they were among the first to use camels, which gave them greater mobility and striking potential. For seven years they ravaged the land and impoverished the Israelites. Then the Israelites cried out to God and Gideon was raised up to deliver them from the Midianites (see Judges 6-8). Not only did Gideon route them in battle, but he also pursued them northward, captured two kings, and dispersed their army. Never again would the Midianites threaten the Israelites.
Though later prophets would talk about Midian, there is no specific extra-biblical documentation relating to the Midianites. For this reason, it is very difficult to determine the exact boundaries of their territory and an exact history of its people.