Need advise- babysitting job

So I have been babysitting since I was 12 and I have never run into this issue. I just started a new babysitting job two weeks ago for a 17 month old boy. I have been bringing my 2 and a half year old daughter with me. I obviously expected some crying from him as his mom has been staying home with him. The thing is he was fine with me from day one. It’s my daughter he has a problem with. Literally anything she does he cries. She will just talk and he will burst into tears. Even if she tries to give him a toy or play he cries. She has been so nice to him so it’s not like she’s been mean or anything. The mom says he hasn’t ever been around little kids except for the two months he was in daycare back in March. I just find it extremely weird and don’t know what to do. I want to give it time but at the same time I can’t take the crying all day. Any advice?