Speculation about Williams' 2020 driver lineup has arrived "too early".
"We're not thinking of replacing Robert this season," Claire Williams is quoted by Spain's El Mundo Deportivo.
"The speculation is frustrating, because unless you really know what's going on inside a team, you do not know what's going on," she said.
"Our relationship with Robert is as good as ever. He is focused, doing what he has to do and we're trying to help him by giving him a better car.
"We're not frustrated at all," Williams insisted.
"I'm sure he is a little frustrated because he doesn't have what he would like to have, but we're only in the middle of the season."
As for 2020, it is clear that Williams wants to keep the Mercedes development driver Russell next year, but may be looking at alternatives for his teammate.
One driver linked with Kubica's 2020 seat is Sergio Sette Camara, a Formula 2 frontrunner who is strongly backed by former Williams sponsor Petrobras.
Williams said: "It's too early to start talking about 2020. We are not yet in a position to make any decisions. And that won't come until the third quarter of the season."