Why can’t Sally play?

M: Hi, you’re new here, aren’t you? What’s your name?
W: Sally.
M: Want to come and play?
W: I’d like to, but I can’t, My mum sys I’ve  got to stay and help her. It’s my brother’s birthday and we’re having a party, too.
M: Oh, you’ve got a brother, have you?
W: Yes, not very far away.  Cathy, my sister-in-law, works in the bookshop over there. They’ve got a new baby called Liz. I mean Elizabeth really, and we call her Liz.
M: Is that your mum over there calling you?
W: No, that’s my Aunt May, my mum’s twin sister. I’d better go. Bye.
M: Bye.