Dead sisters son not interested in property. How do I inherit it?

As you have mentioned that the deceased have left their property in the name of their son, so legally he is the owner of the property and he can dispose off the same in any manner he wants. And even if the deceased have died intestate, as per Hindu Succession Act 1956, a son is a Class I heir while a brother and sister are Class II heirs and the property devolves to any existing Class I heir and only if there is no surviving Class I heir that the property devolves to a Class II Heir.

Section 15(2) (b) of Hindu Succession Act 1956 also says wherein in the absence of any son or daughter the property shall devolve upon the heirs of the husband. If there are no heirs of the husband in Class I, then the property will go to Class II heirs. In case of no Class II heirs of the husband, the property will devolve upon Agnates, and if no Agnates then upon the Cognates.

Applying these rules to the facts mentioned by you, the son of your elder sister is the legal owner of the properties and he does not need to specifically claim the same as they by default would be passed on to him post your sister and her husbans' demise.

We would also like to know in detail what do you mean when you say that the son of your sister is not interested in claiming and owning the said properties? Is he even aware about his inheritance in the first place? Is he not contactable or does he not care for his share of property? Is he Ok to pass on these properties to you and to other legal heirs apart from him? Is he aware that his share of properties is in the possession of unrelated 3rd parties? 

Based on answers to the above questions and any further details that you may share, you can file a suit for possession for the said properties and make the son of your sister a party to the suit.

Any other legal remedy can be advise once we are apprised of the details as mentioned.