What can I do to prevent ethnic conflicts in research groups?
One of the advantages of being the head of a research group is that your position should give you a great opportunity to set the tone of the working environment for the group.

The first key to establishing a healthy research group is setting a good example. Try to be open about your views and encourage good interactions, and be careful not to implicitly support bad behavior by appearing to condone poor treatment or opposition to ideas based on who they come from rather than their merits. Your group will take cues from your behavior, which to some degree will implicitly define what is and isn't acceptable in your group.

Try to be proactive about including everyone.

Make sure your students and researchers are aware that one of the minimum requirements for participation in the group is that people treat each other with courtesy and respect. Unless it's a political science research group, politics and conflicts between ethnic groups should be tabled. It's understandable for people to have differences, but you should try to make it clear that your group is not the place to hash out wider conflicts.