The gay younger brother of a Nationals senator has labelled his sister’s comments against same-sex marriage “extremely hurtful”.

Alastair McKenzie said Victorian senator Bridget McKenzie’s public comments this week opposing change to the Marriage Act were painful to read.

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“As you can understand this is deeply personal and the latest statement from my sister has been extremely hurtful,” Mr McKenzie wrote in a letter published in the Bendigo Advertiser.

“Public statements promoting inequity only serve to perpetuate the isolation and marginalise those youths questioning their sexuality.

“Growing up in the country can be tough and isolating, growing up gay in the country in the 1990s was horrific.

“Given her own story and connection, I had hoped to see a more courageous and compassionate response.”

Senator McKenzie is not commenting on her brother’s position.

“Since entering the Senate in 2011 I have worked hard to ensure my family life remains separate from my political life and therefore will not be commenting publicly on this matter,” Senator McKenzie said in a statement.

Three of her Nationals colleagues this week became the party’s first parliamentary members to openly support same-sex marriage: Lower House Nationals MPs Darren Chester and Kevin Hogan, along with Northern Territory senator and Indigenous Affairs Minister Nigel Scullion.

But Senator McKenzie this week reaffirmed her opposition on the issue.

“As a National Party senator, every vote which comes before the Senate is a conscience vote for me and I will be voting with my conscience to oppose any such bill which seeks to legislate for same-sex marriage,” she said.

“I strongly support the recognition of same-sex relationships through civil unions and have long advocated for an end to discrimination based on sexuality, however I believe marriage is a sacred commitment between a man and a woman and should remain as such.

“I have never opposed providing MPs and senators the right to a conscience vote on this matter.”

A cross-party bill to legalise same-sex marriage is expected to be introduced into Parliament later this year and neither the National nor Liberal parties have decided yet whether MPs and senators will be given a free vote on the issue.