Why DD trust Snape

[Tonks]

No: HPFGUIDX 152486

I am beginning to wonder if the reason that DD trust SS is that Snape's 
parents are in hiding at Hogwarts. Heck everyone else is!! And DD would 
have told no one else. LV would think that Snape's parents are dead. 
Snape would only be a double agent if his parents were either really 
dead or cared for and protected very well. Otherwise there would be a 
serious risk for them as well.

Many here have said that Madam Pinch is really Prince, Snape's mother. 
I wonder if Filch is his Muggle father. We think he is a squib, but 
maybe he is just a nasty Muggle in disguise.

Do we know how long either of them have been at Hogwarts? Even if Filch 
isn't Snape Sr. I am pretty sure after HBP that Pinch is his mother.
We see Filch and Pinch together a lot in HBP. We see Pinch get upset 
over seeing Harry with the potions book. It could just be a protective 
librarian, or something else.

Tonks_op