Is a Will and a agreement (signed) between siblings legally valid?

Hello there, I would like to know if a Will and an agreement (signed) between 4 siblings is legally valid?

No independent witness was present at time of finalising/signing agreement other than eldest sibling as executor.

As one sibling no longer wants to uphold to their side of agreement and directions of the Will… He is named on the family home along with my parents (both passed away) and he and I are also named on another property as joint owners.

The direction of the Will was that the family home of parent share of 66% goes to the 3 youngest children (me included as one) equally and the agreement was that he would surrender his 33% in family home in exchange of me surrendering my 50% on the other property.

Hope that makes sense…