Does the FMLA cover leave to care for a sibling?

Yes; but only in very limited and specific circumstances. 

The FMLA recognizes a special relationship where the employee acts “in loco parentis” – literally, in the place of a parent – to a dependent person, or “child.” 

So what does an “in loco parentis” relationship consist of? It requires a more robust relationship than merely caring for someone with a serious ailment. It requires an intention that the employee serve, as a practical matter, in a parental role for the sibling providing day-to-care and financial support. Some other factors can include whether the in loco parentis parent (1) is in close physical proximity to the adult loco parentis child; (2) assumes responsibility to support them; (3) exercises control or has rights over them; (4) and has a close emotional or familial bond with them, akin to that of an adult child. 

The case that discusses this situation is Chapman v. Brentlinger Enterprises (Sixth Circuit, December 13, 2024).