Conflict over a 'common' house?

our grandparents' house is built on my mother's designated shared property or lot among her siblings. All the siblings knew already each of their designated shares when their parents were still living. Before their mother passed away(father passed away first), the eldest among the siblings already started building a house on his designated part of the estate. With regards to our grandparent's house which is built on the lot belonging to my mother, the house has been a common house for the eldest family and ours although both our parents aren't living there. My question is, now that the eldest's family is moving already to their newly built house, and our grandparents has passes already, they still let my mother know that they still have an authority over my grandparent's house even if it's at my mother's shared part. After our grandparent passes, we waited for years for their house to finish and was expecting they are already going to move for good. They took mostly all the furnitures in the house that belongs to our grandparents to their new house and now they still want something from the house like putting some of their excess things to their old rooms from the house etc. I just somehow felt that they never cared for my mother who is going to live there soon, and make the house look like their stockhouse/warehouse...can someone advice me a thing or two? Do they still have the right to do what they want in the house while they already claimed their shared part by building a house into it? I'm kinda puzzle, but i didn't think about much coz we're just family but somehow i saw some lapses that they didn't value my grandparent's house by damping their excess things into it and even to my mother who rightfully owns the land. By the way, each of their shared lot for the land title is still being processed after my grandparent passes away but ofcouse all of the siblings knew already each of their shares.

My mother's family(me and my siblings) are residing abroad but will be planning for vacation there and got a lot of plans to improve the house turning it into an ancestral home. However, since the eldest sibling among them still wants something from it saying to my mother that they still has the right since it is a common house, i might have problem starting my plan on the house. He even told my mother that the only way their right from it will be over is just to destroy the house and make a new one which is unacceptable for me to think about. Now that i have the time in the world, maybe i can set aside time and confront them through talks of my mother's right and due respect without resorting first legally. And yes, the process of their divided land title isn't over yet though, but still i was thinking to solve this matter first through talks if we truly have the right already of the house which i believe my mother has since it's located on her shared lot and the eldest sibling already built a house on his shared part.