In the fifth year of their marriage, Procne expressed a desire to see her sister. Tereus went to go get Philomela and lusted for her. On the way back, he forced her into a log cabin in the wood and violated her. The king warned her to keep quiet, but when she displayed defiance, he cut out her tongue and held her captive in the cabin. When he got back to Thrace, he told his wife her sister was dead.
This is the part where a few plot holes start coming up.
Philomela made a tapestry of Tereus' crime. Somehow Procne received it and was so angry, she killed their son Itys and served it to Tereus. After he ate, both sisters presented Itys head to Tereus.
Tereus gets angry, grabs an ax and chases them. The sisters prey to the gods for help and they all get transformed into different birds.
How did her sister get the tapestry if she was held captive in the cabin? How did her sister get free?