copyright rules

[sandramort.livejournal.com]

I was showing some adorable t-shirt and longies sets to my mother. The shirts had been purchased and the longies decorated to match, then sold as a set. She pointed out that this was probably not legal and that they could be sued. Any idea if this is correct?

Edit: The thought process I was having was to wonder if that is illegal, of I used purchased fabric to applique one of a set of two items, then painted the other to match, would that also be illegal?