There is no evidence towards execution of gift deed therefore you cannot challenge it on the ground that transfer was void. In fact, your stepmother has sold it to her son-in-law thereafter her daughter erected a house and living therein.
The above-mentioned facts show that your step sister has a valid right over the property. If it was your father’s self-acquired property, then he had the right to transfer it by way of gift to any person even his wife. Albeit you never challenged the gift deed as well as execution of sale deed.
In the absence of any pleading as well as any proper evidence, you can not prove before the court that it was joint family property. In Kuppala Obul Reddy v. B.V. Narayana Reddy, (1984) 3 SCC 447; the supreme court has opined that in absence of any proof or proper evidence about the nature of property i.e. property being joint family properties, it cannot be said that it is a joint family property only on the ground that property was being enjoyed by the family members.
Hence, you cannot challenge the sale deed only on the mere statement that it was joint family property. Your stepmother had been in lawful possession and in the due process sold it to her son-in-law invalid consideration. Therefore the transfer of property in favour of her son-in-law is legal and he has acquired absolute right over the property.