To be in stitches is one of many English idioms related to sewing (“la costura”). A stitch is literally “una puntada” but to be in stitches means to laugh uncontrollably (“morirse de risa”).
The comedian was so funny, the audience were in stitches during the whole performance.
Other sewing-relatd expressions include:
To be cut from the same cloth – “cortado por el mismo patrón”.
The boss’s son is cut from the same cloth as his father. They do things in exactly the same way.
Have / get pins and needles – “sentir hormigueo”
I get pins and needles when I am on the computer for a long time.
To hang by a thread – “pender de un hilo”
Many animals’ habitats are being destroyed and their survival hangs by a thread.
A common thread – “denominador común”
Many customs and traditions in different parts of Europe have common threads.