In the wake of the recent announcement that her half-sister Chynna has entered rehab for an anxiety problem, Mackenzie Phillips tells In Touch exclusively that she will remain by her side. "I love my sister so much, she was there for me and I am here for her," the former One Day at a Time actress, and rehab alum, says. When Mackenzie came forth publicly with the shocking revelation that she once had a sexual relationship with their father, the late singer John Phillips, Chynna was a pillar of support. "I pray that we all get closer through this crisis," the Wilson Phillips singer told Oprah Winfrey at the time, despite some family members' denials of the accusations. Mackenzie, who is currently battling her longtime drug and alcohol addiction on the third season of vh1's Celebrity Rehab with Dr. Drew, is not worried about Chynna's condition. "She is so very strong and funny. She is going to be just fine, I know it," she adds. Chynna entered an undisclosed treatment center last weekend "with the full support of her family and friends," a spokesperson for the star says.