Agree to disagree?

Why does everything have to be a contest over who is right and who is wrong? Why cant it be something that we can agree to disagree on? My sister and I are having a "huge" feud right now. Many of our family memebers had noticed, and mentioned to me, that she tends to favor one child over the other. Shows one more affection, gives them more time, works with them more, gets them involved in activites, brags more, etc. I had also noticed it and was trying despirately not to mention anything because I feared the fallout that would occur. Well, there was a straw that broke this camels back and I said something. The points I made were this...
~ Show more love and affection to one child over the other
~ Leave one child left out
~ Don't brag about both kids equally
~ I know she loves them and is a good parent and wouldnt want to be doing it so that is why I said something.
Somehow, in her mind, it got turned into this...
~ She doesnt love her children
~ She treats her second child like crap
~ I said something to intentionally hurt her feelings

I really wonder how what I said could have gotten twisted so much. It certainly isnt at all what I expected. Then at many times I was told (by her) that i shouldnt have said anything, that it isnt my business. Somehow I just dont see how the well being and feelings of my nephews arent my business. So now we are at a stale mate. She can't move past it unless I apologize for what I said and that isnt going to happen. I have apologized for hurting her and what all of this has turned into, but I wont apologize for what i said because I still believe it to be true. Basically, she has to "win". I need to say "you are right and I am wrong" in order for her to be ok with the situation. There is no "agree to disagree" for her. The main problem of all of this is that my mom is involved too. Sis called her to biach about what I said and Mom told her the same things. And of course Sis had the same reaction to Mom that she had to me. And, of all people, my mom is the LAST person to intentionally hurt someone and Sis knows that. But, Sis still defends that she doesnt favor one over the other and "knows in her heart we are wrong", so I guess I dont get what the issue is. Why cant she just accept that we have different opinions and move on?

Did I cause a big feud with my sister and mother when they said that M and I do too much therapy with O and they don't agree with it? No, I respectfully disagreed and continued to do exactly as I thought was best for my child. I appreciate that they are looking out for her best interest and felt comfortable enough to say something to me about it.

Did M cause a big feud with me when I said something to him about how he doesnt pay as much attention to E as he does to O? Nope, he thought about it and even though he didnt agree, he made an effort and now realizes that it was an issue. E's behavior has changed dramatically.

I guess the only area I can say I was truly "wrong" in this situation is that I thought my sister was a mature adult and could handle the observation with dignity. Boy, was I way off base or what?

Again, why cant she just agree that we disagree on this topic and move on? If she honestly feels in her heart she is right, what does it matter what other people think?