Less than two weeks ago, she was pictured delivering a letter to Kensington Palace that contained a plea for Meghan to make things up with the retired lighting director, 74.
Meghan and her father have not spoken since the royal wedding in May when he was forced to pull out after suffering a heart attack amid a staged paparazzi photo scandal.
But Samantha told DailyMailTV that the baby news has changed everything and is now calling for the whole family to stop fighting and unite around her future niece or nephew.
She said: "I think everybody really just needs to come together and realize this is really important and special for Meg, for Harry, for the child, for all of us, and we need to stop the sparring.
"I want things to be alright in my family and everybody has to be on the same page. I want that for the child but I want that for Meg too."
Despite her highly-publicised grudge against her sister, Samantha said any ill feelings she had "all went away" upon learning the news.
"A baby is incredible. You can't say anything negative. You can't feel anything negative. It's just a beautiful, amazing thing. So my reaction was, 'screw all this stuff and everything that's been said.'"
Although she is still hoping that Meghan and her father will reconcile, she told DailyMailTV that the rapprochement has to happen "in their own space and time" and said Markle Sr is "excited" at the prospect of becoming a grandfather for the sixth time.
"He is excited and I'm not going to let the media take any of that away from him or from Meg β they'll work it out in their own space and time," she said.
"I really want him to be a part of the baby's life because my dad is so funny, he was so funny with us when we were kids so I think the baby would miss out on not being able to meet grandad and I know he's got a huge heart and wants to be involved.
"Hopefully time will pass and this will all resolve itself and be a wonderful thing for everybody."
The news that the Duke and Duchess of Sussex are expecting their first child came early on Monday morning, via a statement from Kensington Palace.
The announcement read: "Their Royal Highnesses The Duke and Duchess of Sussex are very pleased to announce that The Duchess of Sussex is expecting a baby in the Spring of 2019.
"Their Royal Highnesses have appreciated all of the support they have received from people around the world since their wedding in May and are delighted to be able to share this happy news with the public.
In a separate statement, the Palace revealed that Meghan's mother Doria is "very happy about this lovely news" and "looks forward to welcoming her first grandchild".
Although Markle Sr was not included in the announcement, he is understood to have sent his youngest child a congratulatory letter, in which he told her "you will be a great mother".
Other members of the Markle family have also congratulated the parents-to-be, with Thomas Jr telling DailyMailTV that the baby news is "really awesome".
He added: "I'm really happy for Harry and Meghan and I just hope our family can be involved in some way."
Samantha too says she would like to be involved in the new arrival's life, telling DailyMailTV: "I'd like to be able to see my niece or nephew and certainly communicate and send gifts and cards and whatnot, visit on holidays as much as I can.
"But we're so far away so however I can be involved with the baby, I would love to. But more than anything, I want them to be happy."
Both Samantha and Tom Jr say that Markle Sr deserves to play some sort of role in his new grandchild's life and Samantha says the news will have a 'sobering effect' on the rest of the family.
She said: "The baby will be intelligent, funny, cute as a button and I think really funny and loving between Harry and Meg.
"I really hope my father gets to be a part of that but I'm so excited for them and I think the baby will be a beautiful addition to the royals and the Markle's.
"You know what, it will have a sobering effect on the Markles, I'm sure, and all this family drama, everybody will just sit down and get really mellow and conservative β wouldn't the world love that?"
Addressing her younger sister directly, she apologised for some of the things she has said in the past, and said sorry for the "tension and confusion and misunderstandings".
She told DailyMailTV: "I'm so excited, I'm so happy for you and I really, really am sorry for any of the past tension and confusion and misunderstandings and asinine things I've said when I just didn't understand what was going on and I wanted things to work out for my dad.
"But this is just a great time for you guys and Meg, I love you. Congratulations to both of you. Let's just let it all go and you two be wonderful happy parents β I'm so happy for both of you."
