The facts matter, and you have shared essentially none of the facts relevant for us to figure out the status of the divorce case (or if this is a post-divorce action), why there is a trustee, or the trustee's duties. You have not told us if the trustee controls the property because your ex- created a trust and transferred the property into the trust, or if the trustee was appointed by the court to manage certain assets. You have not explained whether your mother made any appeal to the court to try to get the property rented over the past four years. You have not told us the cause of the fire, or the extent of the damage and repair cost relative to the value of the building. Without the key facts, all we can really do is suggest that your mother talk to her lawyer.

If the property was transferred into a trust, with an independent trustee responsible to manage and maintain the property, the trustee would typically have a duty to find tenants when it is reasonably possible to do so, and to maintain insurance on the premises. As we know nothing about the trust or trustee's role and duties, we cannot know if those responsibilities were assigned to the trustee.

If the trustee is not responsible to provide insurance, or the terms of the court's order and trust are sufficient to make the trustee and your mother's ex- jointly responsible to insure the property, with your mother's ex- having been assigned by the court 100% of the control and responsibility for the property, its upkeep, maintenance, etc., then your mother would have an argument that her ex- should be liable for the losses caused by the fire due to his failure to obtain insurance -- whether individually or jointly with the trustee. But without the facts, we cannot tell you whether the ex- has any liability.

In terms of the damage to the property, the loss does not transform a joint asset into your mother's sole asset, but she may be able to get the court to order her ex- to bear the cost of repair, or if the property is sold to assign her a share of the proceeds of the sale consistent with what she would have received had the fire not occurred.

Again, your mother needs to have a discussion with her lawyer.