My sister was dressed with a blue coat.
My sister was dressed in a blue coat.
Use be dressed in + clothes or colour to describe what someone is wearing: ‘The children were dressed in white.’ ‘He was dressed in a warm jacket.’ Do not use with in this pattern.
You can also say ‘My sister was wearing a blue coat.’
When you do not mention particular clothes, say ‘get dressed’: ‘I got dressed and went downstairs.’
See WEAR