My Sister Was Taking Zoloft As An Anti-depressant And Was

My sister was taking Zoloft as an anti-depressant and was prescribed Oxydcodone (Vicodin?) for pain aftr knee replacement surgery. After 4 days recuperation at home, she allegedly committed suicide via a gunshot to the head. I guarantee you she had not planned to commit suicide. She was healing nicely and had experienced good progress in her recovery. Would the combination of these two drugs have been deadly enough to cause her to commit suicide? If so, how much would she have had to have taken to cause this type of reaction? She was 73 years old. Thank you