The Definition of Happy

I think the definition is pretty obvious. What she is saying is that she is sad because she has lost a child and almost lost another one to suicide. Her brother Ward is happy because he has never experienced any of that. So until he walks a mile in her shoes, so to speak, he will never understand what she's been through.

My parents lost a child beforw I was born, it is a terrible loss. For my parents what kept them together and helped them move on was the fact they already had a child, my older brother, and they turned to him, and kept themselves together because of him. It never fully heals it's true, I have talked about this with them many times, but for them having another child, spending time with him and devoting themselves to him was a source of happiness and healing. My birth was a bit more difficult as it brought out their fears of losing another child, but you know, a cute little baby can change your mind on that, and my brother adored me when I was born.

As for Beth, her brother's offer may have been empty or maybe he would have done everything he can to help her, Calvin also consistantly offers his help and wants to ne there for her, the only reason she is "on her own in this" is because she refuses to accept help, or open up.
With her rant about happiness she stops the conversation dead. She decides and in that way communicates that they have no right to speak or be a part of the conversation, allowing her not to deal with her feelings or actually opening up to someone. I personally wouldn't back off there but from what I've come to see people usually get scared and weird around mothers that lost a child, especially if they are parents themselves.

P.s. In one of my psychology classes we went to a clinic and talked to a mother that had recently lost her daughter, most of the students froze and half of them were crying along with her. Taking everything she said at face value (like saying her son isn't important because he isn't aware of what's going on, but the son is 9 years old) and mostly incapable of questioning some of her perspectives or even noticing aditional problems in the family.

This line of the movie seems like one of the rare occasions when Beth was being genuine and heartfelt. Most of the time she is very superficial and maybe even a little bit phony.