I mean, even an TTL half sibling of Muhammad born to a Qaynuqa woman could end up quite similar to our Prophet Muhammad. What defined Muhammad more than his parents was his lack of parents, since he got orphaned at a young age when his mom dies. He never even met his father, as Abdullah ibn Abu Talib died returning from a caravan trip as his pregnant wife Amina gave birth to Muhammad in Makkah. It's a fool's game to try analyze long-dead people psychologically, but you could make the case that the deaths of his parents and subsequent death of the grandfather who cared for him deeply impacted Muhammad - maybe being the reason why Islam became so concerned with the plight of the poor, slaves, orphans, women and other social outcasts.
In any case, if you want a softer POD, you could have Khadija's cousin Waraqa ibn Nawfal be Jewish instead of Christian like OTL. When Muhammad runs home after the terrifying experience of receiving the first revelation at the cave of Hira, his wife Khadija consoles him and asks for her cousin to hear his story because she thought it sounded like the scripture stuff he was always going on about. In the traditional accounts, Waraqa is the one who tells Muhammad that the multi-winged massive alien being of light he saw in the cave was the Archangel Gabriel and that God had given him revelation. A Jewish Waraqa might instead tell him that he's meant to redeem Judaism, not preach a new faith.
That could be pretty interesting, though I am wondering what direction Muhammad might try and take with that Judaism. Maybe he will try and reform it with the Rabbis or something