Who is able to contest a will under the Wills Variation Act? 

The court may vary a will in favour of "spouse or children".   

Spouse: 

The definition of "spouse" found in s.1 of the Wills Variation Act ("WVA") provides the definition of a person who: 

(a) is married to another person, or

(b) is living an cohabiting with another person in a marriage-like relationship, including a marriage-like relationship between persons of the same gender, and has lived and cohabited in that relationship for a period of at least 2 years." 

Therefore, married spouses, separated married spouses who were married at the time of death, and common law spouses of the same or opposite sex may bring claims.   

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UPDATE:  When the Wills, Estates and Succession Act  ("WESA") comes into force, it will make the following changes relating to spouses:

s.2(2) of the WESA:  A person ceases to be a spouse if: 

(a) in the case of a marriage, 

          (i) they live separate and apart for at least 2 years with one or both of them having the intention, formed before or during that time, to live separate and apart permanently, or

          (ii) an event occurs that causes an interest in family property, within the meaning of the Family Law Act, to arise, or  

(b) in the case of a marriage-like relationship, one or both persons termiante the relationship.   

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Children:

"Children" has been found to contain a natural child, including an adopted child, but a stepchild does not have such standing." (McCrea v. Bain Estate, 2004 BCSC 208) 

What if a child is born to a married couple but the man is not the biological father?   

The man may sign the child's birth certificate and immigration card but the court found that such a child does not have standing for a claim under the Wills Variation Act (Peri v. Doman Estate, 2011 BCSC 273).  The court left it up to Parliament to expand the definition of children and DNA testing may be valuable in such situations.