Hurtful situation with my sister

[Tatat]

My dear sis (also my best bud although we're not in each others pockets) is splitting up with her dp of 8 years whom I also adore. We've shared family holidays- they have 2 dc, us 1- social stuff etc all through this time.
I've tried to keep an even view of stuff as it's not my place to judge and I know that she needs a shoulder to cry on. I've partially succeeded. Have let her know that I'm here for her and that whatever the outcome of their situation I love her lots.
However I can't deny I have my own opinions on the subject and believe my sis is treating my BIL sooo shabbily. This is all bubbling under the surface when we talk and is getting harder and harder for me to rein in.
Tried talking to her a bit about it earlier, but got the message loud and clear that she doesn't want to hear any thoughts on the subject of their break up unless they are in support of her point of view. I have said that people are allowed to have different opinions on stuff, it doesn't mean that they don't like or love the person they are talking to. She disagrees and thinks that people (i.e. me) aren't allowed to view their thoughts on a subject (i.e. their split) if they don't agree with what that person is doing.
So basically I can't talk about it to her now unless I am going to be a silent listener, someone to say "ah" and "oh dear" at the right points. In effect, we are just not going to be able to talk about it at all. Which is ridiculous.
But on the other hand, I do know that my opinions on stuff aren't really important in the big scheme of things. So why should I be worried if she doesn't want to hear my point of view?
Am really upset about all of it, I can see her not talking to me again for ages now.(she does know how to carry a grudge does my sis and is a bit of an ostrich, i.e. wont deal with issues head on but just ignore them which is the reason they are splitting up to boot). Even though we left the conversation with me saying (and meaning) "you're right my thoughts on it aren't important", then both "saying speak later love you" etc. Not even sure what it is that's winding me up about it all so much. Am I being selfish in wanting her to listen to me (me me me me me!)
Honest thoughts please, I am not concerned about people having opinions different from mine (unlike my sis)

Aaaargh my head might pop

It's very hard, but I think she needs your support. I've been through a divorce and was amazed by how different friends dealt with it, it really does teach you who your real friends are. I also had a situation a few years ago where a friend made a decision that I found very hard to swallow, but she needed support. It was very difficult to have conversations without agreeing with her and at the same time sounding supportive. I could have lied through my teeth and agreed with her, but just couldn't do that. So, I think you are in a very difficult situation, but if you can do it, then just be there for her and know you've done the right thing.

I think i would do my best to support my sister. If i had as good a relationship as you guys have i wouldn't want to jeapordise that. Also, i would remember that although i'm entitled to my opinion it's not always in my best interest to express it. Maybe when things have calmed down a little your sister will be open to hearing what you have to say. But for now, i would just keep quiet and nod in all the right places

Thank you guys

I am finding it so difficult doing what I believe is the right thing (i.e. soothing noises and a shoulder laden with tissues)- a little support in continuing doing it is really appreciated.

But you are being true to yourself by being evenhanded when it comes to their split.

If they ever end up on amicable terms then she would probably turn to you.

It's hard when you like both, blood is supposed to be thicker than water.

Sometimes sisterly love is hard. You've both been brought up the same but when you have very different views it's hard to see it from the other side