Q. Why do we not call two brothers, or a father and son, to the Torah one after the other?
A. The usual explanation is that this might cause an evil eye of jealousy on the part of the congregation (Shulchan Aruch, Orach Chayyim 141:6). People might say, “How come that the one family is receiving so much attention and honour?”
The Midrash points out that the Torah records an exception to this rule in that the tribes of Ephraim and Menasseh were allowed to bring offerings one after the other (Num. 7:48). Normally the evil eye might have been aroused in such circumstances, but the descendants of Joseph (Ephraim and Menasseh were his sons) were granted a special gift of invulnerability to the evil eye as a reward for his averting his eyes from the enticing wiles of Potiphar’s wife and holding himself back from committing a grave sin with her (B’rachot 20a).
It should be pointed out that the rule against calling two close relatives to the Torah one after the other does not apply if each is called to a separate Sefer Torah such as on Simchat Torah, when one can be Chatan Torah and the other Chatan B’reshit, or where there is a liturgical interpolation between them such as Kaddish. One can be Acharon and the other Maftir.