The reason housed that are upgraded don't sell as well as houses that aren't are often because the decorating person picks colors that appeal to them or what they think buyers might want. Or the value of upgrades and stuff makes the price a lot higher on that house and the other houses in that area just don't sell for that amount.
One of my friends worked his way through college at BYU doing construction work. He built many homes and businesses during those few years. He moved to Oklahoma for a job then didn't get all the benefits he had been told he could have. The house they ended up buying was rather small but in a nice family style neighborhood.
They decided to add on a den to the back of the house, increasing the sq footage and making the house have something the other houses didn't have.
When they decided to move back home a couple of years later they actually found the home value went down because of the addition. I don't know all the ins and outs but the price per foot was less because the house could only sell for a certain amount no matter how big it was because the houses around it would never sell for the same amount.
So if all the neighborhood homes sold for $50K and had 1000 sq feet and this home had 1500 sq feet they could ask whatever they felt was fair but it would still only sell for about $50K because that's what the homes around it were selling for. So the addition was for their own use and didn't increase the value of their home in any way. It was just a cost they spent and didn't get back when the house sold.
I think that right now the housing market is horrible and as long as people are losing their jobs and losing their homes and belongings the housing market will only continue to stay low and fluctuate down there.
I would have an independent Realtor do an evaluation for you with all the stuff along with that, inspections, evaluate the market for the house as it stands and with upgrades, then present that with your offer for them to buy you out. Tell them that if the house sold today, in it's present condition, it would be worth $xxxxxxxx and you are offering for them to buy you out for 1/6 of that value.
I do think that you should all work together to get that amount to an agreeable amount. This is family after all and you should be getting along.