Owner want 4 months to vacate house after selling, Should i buy?

I am buying a house on loan. The cost of house is 30 lacks. I have already paid 5 lacks to them as booking amount. All loan process is complete, only disbursement is pending. But now Owners(3 brothers) want to stay for 3-4 months after registration, because their new house is under construction. If i delay the registration now, then my loan sanction letter will become invalid after 2 months, and after that i again need to apply for loan and complete all the formalities of bank, which is very difficult. And the end date of my agreement with them is on 31 October which is very close. - Should i trust on the seller and buy the house, - or ask them to extend the agreement date and wait for them to vacate the house? in this case i will have to apply for loan again after 3 months. Please help me to take the decision. They are making promise to me that they will vacate the house when their new house construction is complete, And also they are offering me to take rent for the months they will say after selling.