Obtaining home health services with Medicare Part A and FEPB?

I have found most secondary insurances require you have A & B. We were already collecting SS when we received Medicare automatically. When we received our cards A&B were on them. We were not asked to choose.

My husband's neurologist prescribed in-home OT, PT, RN, and Social Worker visits because my husband is a fall risk and has mid-stage Alzheimer's. My husband has Medicare Parts A & B, plus Blue Cross/Blue Shield. Medicare picks up the tab for these services, but they must be prescribed by a doctor in order to be covered.

I have Medicare Parts A & B, as well as a FEHB Blue Cross plan, not a supplemental insurance plan. The private FEHB insurance is secondary to Medicare as I am retired and have no health insurance thru current employment. Are you sure that the family member declined Medicare Part B? Check the Medicare card to be sure.
If the person does not have Part B you should call the insurance company to ask how the coverage will coordinate. You may want to request having a social worker or case manager from the insurance company assigned to help navigate this. Also, be sure that what is actually needed is NURSING care and not just the day to day care of assistance with Activities of Daily Living like bathing, dressing, etc. That won't be covered by Medicare. Check the FEHB insurance coverage brochure for 2022 to see if you can figure out what might be covered. If the family member declined Part B due to the cost (bad choice in my opinion), see if the plan has any type of Medicare reimbursement program. I get $800 a year back from Blue Cross because of the specific plan I have. That eases the bite of the cost of Part B Medicare. If the family member did not originally sign up for Part B there may be a penalty for signing up late. However there could be an exception if they waived it and had other equivalent coverage like their FEHB coverage. You need to find out what they did in regards to Medicare Part B for sure and line up proof of the other coverage for the years they did not have Part B, if you intend to get them covered now. And again for your specific question, be sure it is really skilled nursing that is going to be provided.
Note: Page 21 of this year's Medicare book shows how it works with other insurance.
The brochure also explains that after 20 days the person will have a coinsurance cost for days 21-100 and then then they pay all costs after that. If the nursing care is going to be needed for a long time (21 days) it is really important to hear from the private insurance company what they will pay. And start thinking of really long term planning for a possible eventual need for MedicAid.