I have hesitated posting my thoughts on this, but you asked “What does a true scientific skeptic expect from me?” so I will answer you.
You are suffering from hallucinations. Frankly, you need to see a psychiatrist.
Unfortunately this is the type of answer your going to get here.
Never mind that no one has produced any evidence that ghosts exist. Every rational investigator who has looked into these claims found evidence of misinterpreted natural phenomena, fraud, or just plain old made up stories. Someone who really sees and hears these things is most likely hallucinating.
edited to help get the quotation symbols working properly.
IF her brother was seeing the same thing then there’s not a good chance it’s hallucinations, and she saw the same thing multiple times.
As far as Infrasound the article says that such a thing can make you see theings that are blurry, streaked, or from the corner of their eye, not seeing a clear shot of a garden gnome, that seems kind of specific. But who knows, maybe everyone is different and it runs in families.
I’m not saying anything is true or false, personally I have no idea. Of course I have no evidence it does not exist either. (don’t bother, I’m aware) What a grand circle we could make! ;-P