When does toddler show interest in baby

I have a 18 month old boy and 3 month old girl. When I first brought his baby sister home, he showed no interest whatsoever. No jealousy either. Now, he sometimes gives her kisses especially when he comes gives me a kiss but aside from that he shows no interest in her lol. So anyone with the same age gap, when did your toddler start showing interest in their baby sibling?