Thoughts on asking entire bridal party to wear a cultural bridesmaid dress also?

I am wearing a Thai wedding dress for part of my reception. I really like the idea of Thai bridesmaid dresses but never asked my bridal party (my three sisters, my cousin, and my two future sister in laws) to wear one because I didn't want them to spend more money. However, the bridesmaid dresses they can pick out themselves from Azazie which ranges $60-150 depending on if there is a sale and the style they want. The Thai bridesmaid dress is about $60-80 depending on the style as well.

My sisters came to me asking why aren't they wearing Thai dresses and would love to wear one as part of our culture. I thought it'd be a bit weird to have only part of the wedding party change so I asked my cousin and my two sister in laws. My cousin is Thai so she had no problem. My two sister in laws ignored my text messages where I had to follow up several hours later if they got it and just explain what I'm telling you all now but I am not requiring anyone to wear it.

They just said it was up to me if I wanted them to wear one...when I asked them in the first place if THEY wanted to and if they're comfortable with it but they seem to not want to wear one but will for me. They're Cambodian so it's a similar culture, they can wear the dress to the temple, and they've bought Thai skirts before for Cambodian events so it's not like a waste.

I asked them in the first place to see if they felt comfortable with it and leaving it up to them but they want me to choose for them. I would have them all wear it but I feel bad about the extra money since they're not exactly Thai and they don't seem to want to really wear one.

Thoughts? Is it okay to have part of the bridal party wear the Thai dress?