Ltcg reinvestment in residential property - source of funds

1.  My wife and I booked an under construction flat in Mumbai in Sep 2015 – with first name of my wife, and myself as co-applicant.
2.   The initial amount paid to the builder was Rs 50 L. The amounts were paid as follows: 25 L from my wife’s account, and 25 L from a joint account, where my name is 1st  holder, and my wife’s name 2nd  holder.  The receipts for the entire amount of 50 L, from the builder, are in my wife’s name. (The 25 L paid from the joint account was considered as a loan from me to her, which she has repaid to me July 16)
3.   In July 16, my wife sold a flat she owned, the CG from which works out to Rs 80 L. 
4.   Question:   

Since the CG earned by my wife should be reinvested in residential property in  order to take benefit of the exemptions, would it be correct to treat 50 L, as having already been invested, and only the remaining 30 L still to be invested before July 18?
Or would the IT authorities treat only 25 L as having been invested by my wife, leaving a balance of 55 L still to be put in, in order to avail of the CG exemption available? 
(A related point is, in a joint account of a husband and wife, does the money belong only to the 1st holder of the account, or does the 2nd holder also own this?).

Appreciate your advise, as based on this we would need to take a decision about how much money to deposit in the CG account.