The relative clause "who is sitting next to Peter" is a dependent one.
Btw, in your first example, "sitting next to Peter" is also a dependent clause (though non-finite), so both your examples would (in trad grammar) be seen as 'complex'.
Thank you Paul
Where is the independent clause? Would you point me in a good direction to learn more about this? When or how a sentence changes from simple into complex is a little confusing to me.