The intrusion complex is one of the three "family complexes" which Lacan discusses in his 1938 article on the family, and arises when the child first realizes that he has siblings, that other subjects like him participate in the family structure.
The emphasis here is on likeness; the child identifies with his siblings on the basis of the recognition of bodily similarity (which depends, of course, on their being a relatively small age difference between the subject and his siblings).
"Imago of the Counterpart"
The imago of the counterpart is interchangeable with the image of the subject's own body, the specular image with which the subject identifies in the mirror stage, leading to the formation of the ego.
Formation of the Ego
The image of another person's body can only be identified with insofar as it is perceived as similar to one's own body, and conversely the counterpart is only recognised as a separate, identifiable ego by projecting one's own ego onto him.