Inquiry regarding succession certificate

Planning to buy a property but the apartment is registered in the name of the seller and his deceased person. The seller has a real sister. The seller doesn't has a succession certificate but has a legal heir certificate. When we applied for loan in a public sector bank the manager and their advocate said us that the loan cannot be approved without a succussion certificate. They also guided us that we shouldn't buy the property if the seller doesn't have a succession certificate because this may lead to issues in future. But we are getting a loan from icici bank on the basis of legal heir certificate. The other advocate we consulted said us that in the deed you may include a point stating that the seller should apply for a succession certificate and provide it to us as soon as he gets it. We have already made half of the payment to the seller and paid the stamp duty. Out of the two advocates we consulted one said that we shouldn't go forward without the succession certificate on the other hand one said that we may go ahead by including a clause in the deed which will direct the seller to provide us the succession certificate as soon as he gets it. It takes 4 to 6 months to get a succession certificate. So am confused that wether i should wait for the succession certificate or should go ahead with the deal by just including a clause in the deed demanding the succession certificate when court gives. Please help me out. * The seller is a hindu