Alshich explains that Yisro was worried: Perhaps Moshe would not be interested in a Midyanite woman and would rather marry a Jewess. He therefore emphasized that he brought "her two sons" with him (and not "his two sons"), since a man comes to like his wife because of the children she bears him, and this would persuade Moshe to remarry Tziporah. (See also Maskil Ledovid on Rashi for a very similar explanation)
Rabbeynu Bechaya explains that the way of the Pesukim is to link sons to their mother and daughters to the father. (see Rashi 46:15 "The males are attributed to Leah and the females to Jacob").