Can a valid will be challenged

Yes, if there is a fraud, challenge it in Civil Court.

Yes, will can be challenged on grounds of fraud,coircion and undue influence

Despite the fact experts Mr. N K Assumi and Mr. Bhartesh Goyal has obliged, the query is not very clear to me.

Yes, the facts posted are too vague to offer a meaningful suggestion.

hi sir 

my father had a total property of 494 sq yards. we are two brothers and four sisters. my father passed away in 2018

1. he gave 165 sq yards to his elder son. elder son have registered his property in his name

2. he gave 204 sq yard to his younger son (myself) through a stamped Will. but the property found out to be 186 sq yards.

3. the property adjacent to my property is the third property on which a will has been made in the name of all the siblings. the property is 143 sq yards but in Will it is written 125 sq yards. 

referring to point 2 and 3 my 18 yards ( 204-186 ) has gone in the 3rd property ( 143-125 ).

now I being the younger son is demanding back my 18 sq yards but all the siblings are not in consent with me and instead they are threatening that they will go legal and will take share in my property too becaue it is still not registered in my name.

question1: can they challange my will ?

question 2 : can a stamped will by my father be proved fraudulent. one of the witness in my will is my brother in law ( my sister husband )

question 3  should I go for registerion of my property in my name 

Such discrepencies would not render the Will invalid, if otherwise valid. Once the matter reaches the Court could sort out the discrepenccy based on the doctine of the "intention of the testator".