Does it matter if my Eligible Family Members (EFMs) arrive on the exact same date as me? Would it impact per diem if they arrive later?

Medevac orders are not an entitlement like the Home Service Transfer Allowance. Per Department of State policy (16 FAM 317.1(b)) OB Medevac per diem includes up to 45 days before your estimated due date and 45 days after delivery. If you arrive in the United States prior to the authorized 45 days start date, the additional days before that 45-day window are not reimbursable. Likewise, if your EFMs arrive at a later date than you but within your authorized TDY duration, then their per diem would begin on the date they arrive in the United States. This is because you cannot claim TDY before you are physically on TDY!