"I always used to dress in her clothes."
Translation:Je m'habillais toujours avec ses vêtements.
I am as confused as you are with the prepositions DUO has been using for clothes. Sentence in previous (way back in the levels) module " dressed in a fireman's suit" ... DUO rejected DANS insisted on EN! Sentence in these later modules DUO rejected 'en' insisted on 'dans' for sentence "he came to work in a polka dot suit' ..dans un costume à pois. Now DUO uses 'avec' to say 'dress in her clothes! I did some trial and error translations using the engines systran/deepl/google! inconclusive.
EN is the preferred preposition for what someone is wearing "he came to work in a fireman's suit" should be EN not DANS.
But it seems that in a sentence like this where one item of clothing is not referenced (so the person is not actually 'wearing' the clothes at the time... then avec is ok!
Please any expert help!
"Dressed in a fireman's suit" = habillé en pompier / habillé en costume de pompier. Note that "en" does not use an article in most cases.
"He came to work in a polka dot suit" = Il est venu travailler avec un costume à pois. / ... en costume à pois. Note that "dans" is not the best or even second best option.
"She is pretty in her pink dress" = Elle est jolie avec sa robe rose. Note that "dans" would focus on her body while "avec" is more tactful in that it refers to the overall look of the person plus the dress.
So now, you probably understand why the sentence "I always used to get dressed in her clothes" should best translate to "Je m'habillais toujours avec ses vêtements".