why does Olenna block the willas/cersei match in ASOS?

Initially, she’s trying to get Willas to marry Sansa and get a claim on Winterfell. Afterwards, the main thing is that she doesn’t want to be too dependent on the Lannisters – Margaery’s enough of a connection, and you want to have your dynastic links to as many powerful houses as you can.

Also, Cersei’s unlikely to have many more children.