In monotocous mammals (i.e. where females produce one offspring at a time), most juveniles will experience the birth of a younger sibling in their life. Transition to siblinghood (TTS) has rarely been studied in primates, although it reflects the last step in the shift of maternal investment from one offspring to the next and could thus represent a critical moment for mother-offspring conflict and sibling competition. Here, we use behavioural data on juvenile primates who recently experienced, or not, the birth of a younger sibling to investigate changes in mother-juvenile relationships during TTS in a wild population of chacma baboons (Papio ursinus). We show that (1) mother-juvenile spatial association remained stable; (2) mothers did not decrease their probability to initiate proximity or affiliation with their juvenile; and (3) juveniles initiated proximity and affiliation more frequently toward their mothers, and showed more signs of anxiety after the birth of their younger sibling. Taken together, these findings suggest that juveniles with a younger sibling solicit their mother more often and seek more maternal attention than juveniles without. Overall, mother-offspring conflict could extend in the post-weaning period, during which more subtle maternal resources, such as maternal attention, could be at stake in sibling competitive relationships.