As the property is in the sole name of your wife, she only can give a PoA to her sister. You do not have any title or ownership of the property. Your wife can get a PoA drafted suitably by a competent lawyer, get it either notarised or attested by the Indian Consulate abroad and send it to her sister. It has to be then adjudicated at the land registry in India within 3 months of arrival here. Once the adjudication is done, your sister-in-law can legally act on it.

If property is on wife name she can execute POA in favour of her sister 

the question of husband giving POA does not arise as he is not owner of property 

Yes, both the husband and the wife can execute a Power of Attorney (PoA) in favour of the wife’s sister, even if the immovable property stands solely in the name of the wife. Under Indian law, a PoA is governed primarily by the provisions of the Powers of Attorney Act, 1882, which allows any competent person to authorize another person (called an “attorney” or “agent”) to act on their behalf. Since the wife is the sole owner of the property, she has full legal authority to grant a PoA empowering her sister to manage, maintain, lease, or even deal with the property subject to the powers expressly mentioned in the document.

If the husband also wishes to authorize the same person for matters relating to joint affairs (for example, dealing with banks, signing documents, representing them before authorities, or assisting in property management), he may independently execute a PoA in favour of the wife’s sister as well. The execution of a PoA must comply with the requirements of the Registration Act, 1908, especially where the PoA authorizes actions relating to immovable property such as sale, transfer, or registration. In such cases, registration of the PoA before the Sub-Registrar is generally required to give it legal validity.

Since both husband and wife are residing outside India, the PoA may be executed before the Indian Consulate/Embassy in the country of residence or before a local notary and then duly apostilled/consularized, after which it must be stamped and adjudicated in India under the Indian Stamp Act, 1899 within the prescribed time after receipt. Once these formalities are completed, the wife’s sister can legally act as their attorney in India for the purposes specified in the Power of Attorney document.

POA Act can be accessed online on India Code.

If the intention is to transact including registration purposes for the property in behalf of the property owner then the sole owner i.e., your wife alone can give power of attorney deed in favour of her sister. There's no reason for you to give power of attorney to your sister in law with respect to the property owned solely by your wife because you don't have any rights in the property.

Therefore your wife alone has to authorise her power agent to act on her behalf to carry out the tasks assigned through the power deed.