We have just inherited a house through joint possession with my brothers and sisters. Is it true that we have five years to share?

No. While the Civil Code provides that partition may be requested at any given time, it does not impose a maximum duration.
The co-owners can even decide to conclude an agreement of joint ownership between them relating to the management of the joint goods. Its duration is limited to 5 years, renewable by an express decision of the parties. During the agreement, partition can only be requested for valid reasons.
If the co-owners have not provided for a duration in the agreement, the partition can always be requested, except if the applicant is in bad faith or if the partition is claimed at the wrong time (art. 1873-3, paras. 1 and 2 C . civ.).