According to Halacha, the women’s knees should be covered, that includes when she walks or sits down.
Aside of Rabbi Falks sefer Modesty-An adornment of life(Oz Hadar Levusha) there are organizations as Bnos Melochim which one can call and have their questions answered.
Another issue here is because of openess to the non-Jewish world both audio and visual, some men who are so pulled to the attributes will demand or tell their wives how to dress. Some women do it for Shalom Bayis, unfortunately.
Kindly don’t start attacking me as i B”H am not nogaiya b’duvar. We have children and grandchildren who we are proud of.
Last winter my daughter in law hosted Ateret in her home every Friday night after candle lighting. This is a program for girls to introduce them to the laws of tznius when young. It is done in a fun and enjoying mode.
If my kneehighs will roll down, my 3 year old grandaughter will comment “Babi, you are not tziusdik”. In our family too, the fathers are very makpid on the boys with tznius.
So aside with us women being careful, tznius applies to all of Am Yisroel!
Chazak and Good Shabbos to all.
P.S. Some neighborhood stores that sell women’s apparel have the halachos of tznius posted.