I think it would be abnormal for you to not feel left out! Or hurt!
At best your sister is immature because it has to be 4 grooms and 4 bridesmaids! Lol! I mean God forbid the numbers be uneven! When people mature silly little details like that do not matter all that matters is your loved ones, people being there and everyone being happy! The other factor is she has no empathy for others but again that comes with maturity! How do you think she might feel if you organise a night out at a restaurant with your Mum, dad, brother and 2 sisters and do not invite her? then use the excuse it was because there was only enough room at the table for 6 people! Mature adults would always consider other people and their feelings and would be careful to not leave anybody out. Mindless, immature people on the other hand will always upset others of course!
I find it unacceptable your family haven't protested more! Ok say a sister said something or your Mother cried, are they frightened of the bride to be or something? If it was my daughter doing that on her sister I'd tell her I won't be going to her wedding unless she sorts it out! If she insisted I'd boycott her wedding and take my other daughter out for the day, after all I didn't raise my girls to turn out to be thoughtless, spineless and selfish! I'd expect more from my daughters than that! It sounds awful doesn't it but if my daughter would rather both her sister and me not be at her wedding than go against her idea for 4 grooms, 4 bridesmaids then I've clearly failed as a parent and well there's not much hope for my bride to be daughter is there? I'm sure in that case she would just have to change the dynamics! Yes it's her wedding, it's what she wants but you know what she'll get over it! One day when she is a mature adult she will look back on her wedding day and be so glad a mature, caring adult like her mum made her get her priorities right as in its more priority to inc your sister than have even numbers! Otherwise sadly, one day, when your sister matures and realises how insensitive and hurtful her actions were she will be left feeling tremendous guilt for the rest of her life! You cannot change the past but you can change the here and now! That's why I find it disgusting your parents are not putting their foot down on your behalf or hers! As much as you put on a brave face and try to get over it for being left out you won't! Every time those wedding photos come out or your family start talking about what an amazing time you had it will just open up old wounds time after time, their can be no escape from such a hurt in your life! Your parents should be thinking one daughter might felt deeply hurt for the rest of her life or the other might end up feeling deeply embarrassed, ashamed and guilt once her brain catches up with her actions! Otherwise I am very sorry to point this out but there could be a bigger issue at hand and the need to have 4 groomsmen to 4 bridesmaids is just a lame excuse! I mean if she really wanted you in the wedding party but wanted to keep numbers even she could just get one more groomsman so there are 5 on each side! Or does she have some weird obsession with number 4? Do you think your sister may be jealous of you? It happens a lot between siblings! Yes you all get on but you never know what really goes on in the mind of someone else! She is 9 years younger. Perhaps when she was still at school she saw her big sis with a job, going out with friends and envied you but as time goes on envy could turn to jealousy, my sister is like that. Have you ever had a sense your sister wants everything that you have or just wants to d one better than you' allthe time? Maybe you were the focus of your parents attention as you began to fly the nest which made her feel left out and ever since she's been wanting to get you back? What she fails to realise though is if that was the case you were unaware she was being left out where she is fully aware of that fact! Which is bad! Maybe you are already married, have a good job, look good for your age, have nice things, your sister always wants that so badly too, hence getting married now herself! Or the opposite could be you haven't settled down, your free, doing what you want and she is jealous of that and how you appeal to others and she doesn't want your lovely self stealing the limelight on her big day. You never know if her husband to be gas ever said he really likes you or thinks your good looking, obviously not in a creepy way at all on his part but your sis could be anxious you might steal the attention in her big day when all eyes will should be on her! She might feel inadequate in your presence. Totally irrational as sure everyone will be looking at her and not you but it's hard to get that out of your head if you have always believed someone is better than you.
On the other hand it could be that she doesn't get on with you as much as you think, she might think you won't be fun I. Her wedding party that somehow you will bring the mood down and spoil things! If you have a drugs or alcohol problem she might be scared you will ruin things.
So if you seriously have no issues, as in drug, alcohol abuse or you are known for sleeping around or any other embarrassing, difficult issues that makes your family feel uneasy about you being included and closely involved then your sister is just immature and thoughtless with a hint of OCD and in that case, all the best to her with her marriage, she's gonna need it, or she is jealous of you and doesn't want you involved incase you steal the show!
Hope this helps!
Ps: it won't do any good to ask her about it, her motivation for leaving you out, if she is immature and thoughtless her answers to your question will be also thoughtless and immature. If it's because she has OCD and obsesses with even numbers you can't do anything about that!
If she is jealous of you she will just lie to you
And if she is ashamed of you she'll also lie toy you to save the peace do there's no point confronting her about it!
The only solution really is one of your other family grow a pair and put their foot down on your behalf.
Otherwise if it's down to immaturity you could try making her see what she is doing like inviting your family, not her then just saying you thought she'd be too busy with wedding plans or there was not enough space for her, it might make her see what she's doing and if she kicks off that you didn't include her don't say 'well now you know how if feels' say instead ' oh I'm sorry I didn't think you would mind and you would understand, you know just like I do about your wedding and decision to only have our 2 sisters as bM and not me to keep numbers even! It might not change anything but at least you know you tried and if she matures one day and realises she'll know you tried to reason with her but she ignored it and her guilt is only down to her actions nobody else is to blame!