How to rate photo relative to others in a set?

You mean other than "be more sparing in your assessments"?

I only ever flag session images with one or two stars. Most images are either culled or receive no stars.
Nothing ever gets a 4 or 5 until it is published & either earns good money or is well-received publicly [depending on publish method]

This method allows a 3-star to be 'hot' but not published.
As my sessions are store by date + session name, that allows me to quickly filter any session by star rating, but I have no method that can filter globally.

About 95% of my work is done on contract for the clients (they pay for the exclusive, unlimited rights to the images), so I don't have a concept of cross shoot comparison in my workflow. Speed is also essential, so I use a multi-pass method to cull my images:

Pass 1 - Pick Phase

This is a very quick assessment, 1-2 seconds per image where I throw away any pictures that have technical issues, I have an immediate negative reaction to or just don't "speak."

The images that are left get flagged and marked with 3 stars. Usually leaving about 20% of the photos remaining.

Pass 2 - Up Or Down

This is a longer assessment, 3-5 seconds per image. Asking if compared to my current average work, is this image better or worse than average.

The images that are about average stay 3 stars, lower become a 2 star, higher become a 4 star. Leaves about 5% the original images at 4 stars.

Pass 3 - The Cream Rises

This is the longest assessment, 5-10 seconds per image. Asking if there is anything genuinely special about the image.

The images that make the cut are increased to 5 stars. Leaves about 1% the original images at 5 stars.

Pass 4 - The Client

Finally, I make proofs and show the 4 and 5 star images to the client. For situations where I make my money off print sales, I will include 3 star images that might be special to the client even if they only represent my average work. (Images of the "grandmother pinning the broach she wore at her wedding on the bride as her something borrowed" sort of thing.)

I use color ratings to determine the reaction of the client.

Red: The client didn't like the image. (Rare after 24 years of working with clients and learning what different types of clients like.) Yellow: The client had a neutral reaction. Green: The client had a positive visual reaction. Blue: The client commented positively on the image. Purple: The client got excited about the image.

I use one of those little 5 color highlighters to mark the backs of the prints based on their reaction and transfer those ratings into Lightroom when I get back to the office.

Using this method, I can sort and rate 500 images from a 12 hour fashion shoot in about half an hour, and the 1200-1400 shots from a wedding in about an hour and twenty.