Any verbal discussions at the time of marriage or among relatives would not have a legal hold or value.
Your father-in-law's second marriage and the child would be legal as it happened after the death of the first wife.
The second wife and her off-springs will have rights to the share of the property that your father-in-law has currently.
Your husband or you cannot argue on the grounds that it was a secret, not accepted in the family etc.
Seek the help of a good lawyer in your area and then explain the case, the age of the second wife, the offsprings and their ages. The circumstances related to the second marriage was it held at like a ceremony accepted in your community or was it a registered marriage. Then the details of the properties in the name of your father-in-law and his father.
My suggestion would be, such issues happen in some families wherein a quasi-legal settlement is arrived at by all parties after a few meetings and discussions between both parties in the presence of family elders, local leaders etc.
Once such a settlement is discussed, your husband would have to agree to share the property. Your father in law is alive. If your father-in-law makes a valid legal will giving everything to the second wife and her daughter. Then all you can do it only appeal to it. SO, better would be a negotiation or a compromise mediated by elders.
Assuming that you are Hindus, Your husband also should remember that prior to 2005, only sons were coparceners (by birth, have a right to father's property) after 2005, even girls are considered as coparceners and can ask for a share of the property even while the father is alive.