In which State and in which city is your building? Did the building construction come under RERA regulations? Is there a separate pump and overhead tank system for each floor or each flat? I presume the following. Tell me if I am wrong.
There is building wth ground + 3 floors. There is one flat on each floor. Each flat(member) including you, has an overhead tank and pumping system. The overhead tanks are on the terrace or above the terrace. All members have right of passage and access up to the tank. It means that they have right of passage through the terrace also to approach their respective tank.
Do you exclusive right over the entire terrace or an earmarked section only?
Unless there are separate tanks and separate pumping system your compalint cannot be about 2nd floor member only.
A common terrace to which all members have right to access cannot be sold to one member only. It creates many ambiguities. The reply of the member is valid. It appears to me that the selling of the terrace to you is like sellng Howrah bridge or the Eiffel tower to a private party
Anyway your right or otherwise over the terrace has nothing to do with the validity of your complaint. Your problem is difficult to solve especially since you do not have an association.
If you go to court it is even possible that all the other members join together and pray to the court to undo the sale of the terrace to you.