If the house and lot is part of the estate of the deceased and is unpartitioned, it is co-owned by you and your siblings. Any improvement on the property borne by only one co-owner does not confer ownership to him.
In your case, you allowed the improvement, so it automatically operates that all of your siblings is obliged to pay your older brother your pro rated share in the improvement.
For the 30 years prescription to operate, there must be open, continuous, and adverse possession of the property (sorry if I can't remember the verbatim text of the law) and if a person is aware of the co-ownership. he cannot declare adverse possession.
This is what I remember, hope this help and I am asking for other member to shed some light on this case because I am a bit rusty on this aspect