1) it would have been advisable to register the partition deed for division of house by metes and bounds . unless it is duly stamped and registered partition deed is in admissible in evidence
2)to carry out additional construction you need consent of other legal heirs .
3) if BIL has expired his legal heirs ie his wife and children can give consent .
4) if second BIL is asking for additional money pay him to get his consent .
5) before proceedings further get your house surveyed by structural engineer whether house can bear the load of additional floor ?
6) get plans sanctioned by muncipal corporation and then only carry on additional construction