A bit of a rant...

[cessna152towser]

I wouldn't worry about her opinion. Without wishing to come over as insulting to your sister, from the way you describe her I'd say she has her own opinions on dress sense, both on how to dress herself and on how she thinks others should dress. We are all entitled to dress as we choose, even if in the eyes of someone somewhere we might look silly. She could benefit from keeping her opinions on your dress sense to herself unless she's willing to reciprocate and let you comment on her clothing. You might offer to give her your frank opinion before actually telling her that you equally think she looks silly.
It is good that your father and your other sister are supportive but you can never please all people all of the time.
I have two denim Union kilts, one light blue and one dark blue, both of which I wear quite regularly and have never heard any adverse comment either from family or strangers about wearing them.
Yes they could be called denim skirts for men and they are a long way from the concept of a traditional kilt which would placate the tartan police, but they do meet the criteria of kilts in having pleats at the back and overlapping front aprons.

[acstoon]

Tough as it is... ignore it... stubborn ignorance is tough to combat.
I have a sister-in-law who, despite the fact that many in my family are kilted, is rather condescending/patronizing regarding kilt-wearing. Small minds. There's a general assumption that relatives' input/opinions/advice is automatically so much more valuable... wrong! Much as I love my family, I am aware they are not all-knowing, and as fallible as the next person. Therefore, I do not feel I must endure, nor heed their words.

My personal (and very uninformed) opinion: Jealousy. You (and your father) wear a kilt that not only reflects your history, heritage, and self-confidence... but you also look damn good in it, too.

I've never been a fashion-monger. To those who have admonished my fashion sense, I have a standard reply:
"I dress for comfort, within reasonably acceptable social norms. I do not feel I need to follow trends established by people, who live in far away places, whom I've never met, and hold values that are questionable to me. If you wish to purchase me another wardrobe, you may do so... I MIGHT wear it."

All I can humbly suggest: Quietly smile at her ignorance, go about your business, and lead by example.

[caig]

Arlen, you are absolutely right to wear what you want, when you want. It is really obnoxious for any person other ( related or not) to tell you what to wear. My impressin is that you will ignore this person and go with your own intuition. Good on you.

I live in a "hard" working class area of North East England and I get a bit of friendly banter from people in my local pub, but NEVER get any insults or anything like that.

Keep it up.

By the way, I have a blue denim kilt from Kiltstore, which is just fabulous.