Kardashian and Thompson have one child, daughter True, who turns 3 in April. Kardashian confirmed this month that she is undergoing IVF in the hopes of having another baby.
On Ellen, Jenner said she's thrilled about Kardashian and Thompson's plans.
"I think it's so great that they want to do that again and have a sibling for True, because there's nothing better than a big family, and brothers and sisters, and that's just my heart," Jenner said. "So I'm really happy."
"I just want them to be happy. And True will be so excited," she added.
Jenner also addressed a photo that Kardashian posted on Instagram in February that showed her wearing a large diamond ring on her left hand. The picture sparked engagement rumors.
"I think she just loves jewelry. She loves a good, you know, ring," Jenner said. "You should ask Khloe that."
E! released a trailer for Keeping Up with the Kardashians Season 20 in January that shows Kardashian telling Thompson she is ready to have a second child.
Kardashian confirmed on Thompson's birthday this month that they are back together following his 2019 cheating scandal.
"The ones that are meant to be are the ones who go through everything that is designed to tear them apart and they come out even stronger than they were before," Kardashian wrote on Instagram.
"Thank you for showing me everything you said you would. For the father you are. For the best friend I have in you," she added. "I hope you know today and everyday how loved you are by me and so many."
Keeping Up with the Kardashians is in the midst of its 20th and final season.