The original text apparently had a case of prepositional repetition in this verse (“in turning your backs upon the poor and the needy and in withholding your substance from them”), but scribe 2 of 𝓟 accidentally omitted the repeated in when he copied this passage from 𝓞 into 𝓟. Oliver Cowdery caught the error in his proofing of 𝓟 against 𝓞. Either reading is possible, as can be seen in the following similar example involving the preposition in and a sequence of conjoined gerunds:
In this passage, the preposition in is repeated only in the first case (“yea in making fortifications”); in the last two cases, there is no repetition (“yea and also delivering … and providing”). We therefore follow the earliest textual sources in determining whether the in should be repeated. Oliver’s inserted in in 𝓟 for Alma 5:55 was most probably the reading of 𝓞. The critical text will assume as much. For further discussion of the repeated preposition in, see under Mosiah 24:1 (as well as more generally under conjunctive repetition in volume 3).
Summary: Maintain the repeated in that Oliver Cowdery supplied in Alma 5:55: “and in withholding your substance from them”.