Sandra Parsons told The Sun that she is ecstatic to be having a grandchild.
She said: "We're thrilled and she is unbelievably happy."
Brook, who has been dating model Thom Evans since November, announced that she is expecting via Twitter on Wednesday, writing that she is "delighted" at the news.
Parsons added: "You'd think they'd have a beautiful baby with their genes."
The 31-year-old's ex-boyfriend Danny Cipriani earlier said that the model will be a "great mum".
Brook has since thanked fans for their messages congratulating her.