My mother wears tight clothes at the areas of the breasts and belly in front of my adult male brothers. She also asks me to iron her dress and to help her by closing its zipper. Is it permissible to obey her in this regard? Is it considered cooperation in sin and transgression? It should be known that I offered her advice before and tried to convince her, but my mother, may Allah guide her, is quite difficult to convince.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If your mother's clothes cover the ‘Awrah (body parts that must be covered) and do not cause temptation, then she is permitted to wear them in front of her adult male children. So you are required to obey her in ironing these clothes and to help her in wearing them. However, if her clothes are so tight-fitting that they show the details of her ‘Awrah and cause temptation, then she is not allowed to wear such clothes before her sons. In this case, you should not help her. Rather, you are required to gently and politely offer her advice and explain the limits of the woman's ‘Awrah in front of her mahram (permanently unmarriageable) men to her. 

You should guard against talking to her in a harsh or impolite way. One's parents are not to be treated like other people when it comes to enjoining them to do good and forbidding them to do evil. In all cases, you are required to be dutiful to your mother and treat her kindly.

Allah knows best.