Mako Vunipola has suggested brother Billy is highly unlikely to be seen in the NFL any time soon.
Last week, the younger Vunipola expressed an interest in trying his hand at American Football, while acknowledging any switch of sport would not happen in the near future.
"He's got to learn the rules first," quipped Mako in an interview with BBC Sport.
"I'm excited for him if he does get the chance, but it's a long time away and he's still got a lot of things ahead of him first.
"So I think he is just running his mouth a little bit."
The Vunipola brothers have each starred for England under Eddie Jones and earned fulsome praise from the national coach.
However, prop Mako is sure he still has much more to offer.
"The biggest message is not to be happy about where we are," said the Saracens star.
"There is more accountability and responsibility on the individual. Eddie Jones keeps sending me texts if I need a nudge.
"We want to be the best we can be. For me it's about improving one per cent in every area."
England face South Africa at Twickenham on Saturday, before further home Tests against Fiji, Argentina and Australia.