I don't think there is anything you need to worry about. May be your sister is happy with that kind of lifestyle. However, you did not explain why that man is backward to you. I don't think those who wear trendy clothes, attend parties and date with men (despite being married) are not modern at all. If you do all that and never pay attention to your family, children and husband definitely you should be called selfish not modern.
I would appreciate that couple because both are paying full attention to each other. This is why I suppose they are leading a very peaceful life. Apparent beauty has to go away sooner or later.