You are better off to talk to attorney.
I don't think one should buy something they expect to inherit. However, in layman's terms, for example If property's value is, let say, $100K and one out of three siblings bought it for $100K AND became sole owner of that house AND money went to parent(s) , I think, other siblings won't be entitled to anything. If that person paid less than 100% to parents, than it looks like sort of a reverse mortgage: when property is sold, that person must get what was already paid plus reasonable interest. The remaining amount should be split between all siblings according to the will.
Again, please consult attorney, as I am not one and cannot give out legal advice.