Sister qualifies for Medicaid. Now what?

My younger sister, age 21, recently moved to New York, where our mother and I live, from Florida. Mom and I both have insurance through our employers; sister is a dependent on mom’s, which is a PPO through United Healthcare with a $800 deductible and $3.9k OOPM. Upon arriving here, sister needed acute mental health treatment, which was unfortunately only available at an OON hospital. We spent weeks looking for alternative options, but there were none. After some fighting with UHC, we eventually got her into the program; the program then recommended that she enroll in Medicaid, as it will open up treatment options for her in the area. My understanding is that she was taken to the hospital’s financial services department today, qualified, and enrolled. My question now is… which is going to be primary for her? Medicaid or mom’s UHC? I’m more than a little worried mom’s will be primary and that we will keep having to jump through hoops to get her the treatment that she needs, despite this program’s best efforts to give her her best shot at getting better.