My parents, who are now deceased, separated shortly after my birth. My father had two other children. I am in a civil union and have no children. A friend told me that upon my death, my assets would be transferred to my sister and stepbrothers. (...)

(...) Is this correct ?

Yes. When a person dies without children, their surviving spouse is entitled to inherit their assets (Article 757-2 of the Civil Code). The term spouse refers only to a non-divorced husband or wife (Article 732 of the Civil Code). Since your civil partner is not a legal heir, it is indeed your brothers and sisters who will inherit your assets (Article 737 of the Civil Code).

Good to Know : Since they are not reserved heirs, you can always exclude them in favor of your partner.