Comlink-X – Try Chapter 75. This is only talking about what you can’t see when praying.
It doesn’t say anything about a chiyuv for women there, Abba_S
(which is what Lilmod Ulelamaid said she knows to be in the M”B).
I don’t believe the obligation to dress modestly is in Mishna Brurrah but the Chafetz Chaim ZT”L probably said it in his writings. I seem to remember reading that he said most of the problem that the Jewish people got was brought on due to immodest dress. The prohibition for immodest dress is based on the prohibition of placing a stumbling block in front of a blind man. Immodest dress results in immoral thoughts to those who see it and a breakdown in society.