Is it considered a sin to take immodest clothes to the laundry, by analogy to the principle that whoever sews for oppressors or sells them what aids their oppression is considered an oppressor or an assistant in sin?

If it is known that the girls adorn themselves and wear these clothes in front of those to whom it is not permissible, then it is not permissible to help by carrying them to the laundromat; for that would be assisting in sin, as Allah Almighty says: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression). However, if they are chaste and restrict wearing them to a concealed place, then there is no objection to that.