Serena Williams will be the mother of a baby girl, if sister Venus is to be believed after she appeared to let the secret slip on Wednesday.
Venus, playing at the French Open in Paris, told a TV interviewer: "She's going to call me her favourite aunt."
"We all like 'baby V, baby Lyn, baby Isha'. We all want the baby to be named after us," the 36-year-old told Eurosport, referring to the names of Serena's sisters.
Serena, who revealed she was pregnant in April, has refused to discuss the sex of the baby she is expecting with fiance, Alex Ohanian, the founder of Reddit.
She was at Roland Garros on Wednesday watching her older sister defeat Kurumi Nara in the second round on the showpiece Philippe Chatrier court.
"I think it's not easy for her to watch, because, you know, obviously she knows she can do well here," said Venus after the match.
Serena is a three-time champion at Roland Garros.
"I think that she's just excited about everything coming up in her life, on and off the court, excited about returning here hopefully next year," added Venus, who turns 37 next month.
Serena, who won her 23rd Grand Slam title in January when she defeated Venus in the Australian Open final, is due to give birth in September, the month she turns 36.
Earlier this week in Paris, Venus had talked about her anticipation of becoming an aunt.
"I'm definitely looking forward to it. I have no idea what the experience will be like, because previously I became an aunt when I was very young and lived far away, across the country, so it was a completely different experience than it will be this time," she said.
"So I hope that I can live up to this job. I've got to get some confidence that I can do this. It's all new for me. It's a great journey."
Venus, who was runner-up to her sister in Paris back in 2002, added: "I think you just have to be ready, aware, and alert. Then have a lot of diaper duty. Yeah. So we'll see."
Serena learned she was pregnant just two days before the Australian Open.
She has told of being nervous about playing, uncertain whether it would be too dangerous for her or the baby.
Kelly Bush Novak, Williams's publicist, said last month that the US star "looks forward to returning in 2018".