Feedback occurs when the signal from the mike goes through the sound board and back out amplified and is then picked up by the microphone again causing an audio loop that feeds back on itself, thus called feedback.

As your sister knows, that is clearly a problem if the mike is in front of the PA's speakers.

There are some other possibilities. If your sister is using monitors, it helps to have the microphones "rear end" pointed towards the monitor so the front of the microphone faces away from the monitor. If the mike picks up sound from the monitor, you have another feedback loop.

Nearby flat surfaces are a problem. Therefore it is best if the guitar mike is not pointed at a ninety degree angle to the face of the guitar. This helps eliminate reflective feedback. Other large flat surfaces can also be a problem. The ceiling or the back wall may be too reflective. It takes some doing then to find the right mike placement. In this case, some change in speaker placement may also help, so that the faces of the speakers are not at a ninety degree angle to any of the walls.

I suspect there are other sources of feedback, but you have just reached the end of my knowledge.

Roger in Baltimore