For problems like this, check the FAQ section in the walkthrough:
Q: What is the solution for the murder crime in Millwater?
A: After speaking with everyone downstairs, you can go up and find a strange potion. After finding the potion you have to talk to Winona, Lady Redwood and then Irinlia. The order is important, since their dialog's affect each other in this order. Afterwards, Ryen can solve the crime by talking to the soldier next to the corpse of Sir Redwood. The right choices are Irinlia – poison – by mistake.