Asked whether being sincere is a state or a characteristic, the student answers that it could be either depending on the situation. That is true of the usage, but it is not how Spanish decides.
Soy sincero · Somos sinceros · Somos sinceras
Your usage is not restricted, though. If you want the sense of being sincere right now, you can say it just as in English: I am being sincere.
estoy + siendo + adjective
Siendo is just ser with the -iendo ending, built exactly like vendiendo from vender.
So the concept stays characteristic, and the structure lets you talk about it as a state anyway. Being honest is characteristic, yet we are being honest works the same way in both languages.
There is no rule here, but the route works. Sincerity gives sinceridad, and from there you can play around and land on sincero.
The main drill of this track. English wants -ing in places Spanish does not, and that is negative language transfer to resist.
Los compro mañana · Los compro la semana que viene · but estoy comprándolos ahora
All three of those English sentences use -ing. Only the last one is right now.
Los compro · Los estoy comprando ahora / Estoy comprándolos ahora
The pieces get reassembled rather than recalled: la semana, then que, then work out it comes from venir.
la semana · que · viene
Los compro la semana que viene.