Advice please

I help my sister out with babysitting. I was collecting the 3 year old from school today and normally he’s okay with me, but today and yesterday he was like don’t look at me don’t speak to me, so i just walked ahead of him but yesterday he started to joke around with me and was grand from then on, but today he literally screamed. He was screaming saying how he’s telling my sister that i’m being bold and i’m saying bad words, i never curse around the kids, i was telling him to get on the path and he was shouting don’t talk to me, i’m telling my mum on you, i was asking him would he want my sister to collect him tomorrow and he was like my mum is collecting me and i said she’s working tomorrow and he started to cry, he then started to scream saying how i hit him and i clearly dont so i was like no i don’t and he kept screaming it and crying, walking up the road like with people around i was so embarrassed, i was telling him im ringing Rachel and he hit me, i walked ahead and when he was running up he fell i went to go help him and again he was screaming saying don’t look at me. I ended up calling my cousin in tears and telling him to drop him back to house. Like i don’t know what to do, i know kids lie but to say something like that in public i’m literally so embarrassed and sad that he’d even say that, cause we get on great, he sometimes is bold and the worst id do is shout at him to stop, my sister normally would deal with him. She needs my help tomorrow but i don’t know if he’ll act the same way again and if he does what do i do to make him stop, and what if he tells his mum and she accuses me of doing it ? i really want to leave but i’d feel bad cause she needs the help with the kids