إِنَّ expresses emphasis. The preposition in لِلْمُسْلِمِ conveys “for a Muslim.” The emphatic lām in لَحَقًّا highlights the right being discussed.
إِذَا رَآهُ أَخُوهُ means “when his brother sees him”; أَنْ يَتَزَحْزَحَ لَهُ means “to shift aside for him.” The verb يَتَزَحْزَحَ conveys moving aside a little.
The sentence connects respect for a fellow Muslim with a modest practical gesture in a gathering.