The former Wilson Phillips star, who is married to Billy Baldwin, was admitted to an undisclosed recovery facility over the weekend (13-14Feb10).
Mackenzie has vowed to stand by Chynna as she battles her personal issues, just as the singer/songwriter did for her when she shocked Hollywood last year (09) with the news of her decade-long sexual affair with their father, Mamas & The Papas founder John Phillips.
And the former One Day At a Time actress is confident Chynna will bounce back from her rehab stint.
Mackenzie tells In Touch Weekly magazine, "I love my sister so much. She was there for me, and I am here for her... She is so very strong and funny. She is going to be just fine, I know it."
Mackenzie's incestuous relationship with her dad, which started after he raped her on the night of her wedding to Jeff Sessler, was exposed in her memoir High On Arrival.