If your relatives are in agreement to sell you the house with stage payments, I think it would be a very good option for you in the long run.
In the short run, I would park the idea of buying the house until the issue of the 1st house has been resolved. If the bank becomes aware of your intention to buy the 2nd house, its likely that they would want to transfer your existing mortgage on to it. That would make things very complicated to resolve now
Under the new insolvency legislation, a person's home is protected in so far as it is reasonable to do so and for that reason I would say that there is very little that the bank can do in relation to the 2nd property if it is now your home.
Of course the fact that you own 1/3 of the 2nd property at a time that you are asking the bank to write off a large part of the negative equity on the 1st house is a hurdle to get over but I think its something that can be negotiated. That is why I earlier suggested accumulating a small lump sum that could be used for negotiating with the bank if that's possible.
I think you need to take this one step at a time. The 1st step is to get rid of a house that you're not living in, that you don't want and that you can't afford. When the bank tells you that they will extend the mortgage term blah blah blah. That is
probably the best outcome for the bank. Its a terrible outcome for you. You want to sell the house. You dont want to live there and I dont think you want to be a landlord. The house must go. Be strong. Be firm. Do whats good for you