This is one of those things that nobody really knows.
It feels like most of us believe twin flames are destined to be romantic and that seems to be how it manifests for the most part. There are people who claim to have a sibling or friend as a twin flame, but it has none of the earmarks of the journey, so it seems to me like a misuse of the term, and what they really mean is a soulmate, but who knows.
There are already several threads on the topic, so I’ll not rehash them:
The most common view holds that twin flames are always destined for romance. They share one soul blueprint, designed to unite fully as both heal and align. Friendship is just a starting phase; the bond becomes teacher, best friend, partner, and lover all in one. If you’re stuck in the friend zone, more healing awaits.