Someone have a registered sale deed without mentioning how sale executor acquired property.

My family had a registered sale deed in 1954; my two neighbors also bought a property from same vendor in 1954. In that 3 sale deeds have 4 schedules, in that 4 schedules 2 (1 and 2) of them are independent schedules of extent of land 2 acers. 3 and 4 schedules are joint schedules of extent of 40 cents. One of the neighbor namely C had 1950 document of extent 20.5 cents land. But this registered document land is not on the ground. On 1984 one of C family member sold 3 and 4 schedules (without knowledge to A and B) to stranger with registered sale deed not mentioning how he acquired titles in the sale deed. By showing our 3 members joint property schedules 3 and 4. In 2022 I bought B member share of 1954 schedules 3 and 4 and I am in possession since we bought land in 1954. Some person has come now and claiming that this 3 and 4 property (40 cents) are belongs to them showing municipality tax receipts. I have two questions 1. Does that sale deed is valid if in that seller does not specify how he acquired title by writing it in sale deed. 2. Stranger filed civil suit on asking possession does it valid selling joint property to stranger without partition and aware to share holders.