I don't think it's an abstract interpretation of the FARs. It's 100% legal and clearly stated. I was in the same boat years ago while working as a line service technician/CFI. As long as you meet the requirements it should be logged.

What if you didn't log it? Then you really couldn't bring it up in the future at an interview. I got a lot of good (and very very bad) lessons in how to be a professional pilot while sitting in the right seat of a Citation SII with less than 400 hours TT. Later, I used what I had learned to make myself a better pilot. In interviews, I had something to talk about besides CFI experience (which i still think is the best experience for a newer career pilot).

If I was in your position to sit in the right seat with your brother I would do it. Learn as much as you can, but like others have said, don't think that your SIC experience makes you a PIC. The minimum training required by FAR 61.55 to act as an SIC is the MINIMUM.