The Wild Things actress, who adopted a baby girl called Eloise in 2011, said that Sam and Lola were instrumental in making the transition a smooth one and have fallen in love with the new addition to the family.
"To be honest, I didn't want to involve them that early," Richards explained to People of how the adoption process affected the 7 and 8-year-olds.
"But when you go through the adoption process you have to do a home study, and they have to talk to [your] children, and I didn't want to talk about it with the girls that early, because it was two years before the adoption came through.
"I wanted to keep it private, and in our family. I'm actually glad, because it turned out to be a good thing."
Richards further praised Sam and Lola for their maturity and helpfulness as older siblings, adding that the experience has affected the children profoundly.
"They want to adopt when they get older and become mums. I'm so very proud of them that they've really embraced Eloise," the 41-year-old said.
"They're so protective of her, they play with her, they love watching her grow and change, and so far I haven't seen any jealousy. They're the best big sisters, but they still haven't changed any diapers."
Richards recently brushed off suggestions she would ever reunite with Charlie Sheen, joking that she is now 'too old for him'.