The problem seems to lie between giving him a piece of your mind (as you stated in the OP) and what you've said above. It's not the same thing.
I come from a family that does not discuss emotions or relationships easily, so I can identify with the frustration of being a person who wishes more could be said and shared. This would enable you to get your DH and his brother and his brother's new love together easily, rather than feeling like you have to become 007 to even find out her name.
But the main issue here seems, at least to me, that you are wanting to spare DH more pain. Rarely getting to spend time with someone so important in his life, coupled with the loss of a pet, has got to be hard, and I know I tend to get more upset over my DH's emotional pain than my own -- I just want to protect him.

Hugs and hopes that you can contain your frustration and try to really open the lines of communication in the family rather than getting angry.