I'm on vacation and my sister has my car while I am away. She sent me an e-mail saying that she notices on the rear rims, streakes of a black oil-like substace (not sure if it's brake fluid)

 

Likely something from off the road. Brake system has pretty high pressure when applied, if it were brake fluid she would have noticed the pedal going to the floor.

Tell her to clean it off and if it comes back let you know, other than that, let you have some peace while you are away from home.

 

wheel bearings?

 

I changed them like 4k miles ago. You really think bearings can be the cause being so new?

 

I doubt it is the bearings.
It might just be a fluke.But it might be like dirty water or something from in the center of the rim.

 

pix of the sludge?