Whether your Husband died intestate ie without writing any Will?
Another question, how your Husband and his Sister acquired the property in the first place? With their own funds or was the property devolved on them jointly from their Father?
If the property was sold subsequent to the death of your Husband and your Husband passed away without writing a Will, to the extent of your Husband's share, you and your children need to get a share of the property. If Children are there, Minor's interest also is involved. In this background noted above, if you have not signed the documents in your capacity as Legal Heir and on your minor Children's behalf, the sale is not valid. You may contact an able and reliable local Lawyer and explain all details to him. You may think of filing a case against your In-laws, Buyer, Registrar who registered the property, and the Society which gave NOC/transferred the share if any seeking cancellation of the sale. Once the case is filed, they shall come for compromise and you may get your share in the sale consideration.
And, it is an open secret that the sale consideration noted in the documents is only as per Government rates and the real consideration may be much more.
If the sale had taken place when your Husband was alive and he also signed the documents for selling, you have no case. However, he acquired property jointly from ancestors, your Children shall have a right on the property to the extent of the Husband's share.
There are many relevant facts that are not disclosed or not available in the query and subsequent rejoinder. Therefore, contact a Local Lawyer and seek his advice for further action giving full details and showing copies of documents if any.