Why did my sister stand up for me when she has made it very clear that she hates me?

[jimrich]

You want us to believe that things were always so good between you and your older sister but if it were true, a little "fight" would not make your relationship so bad now so, it's my guess that things have never been all that good between you and it's gotten worse in recent years.
Generally, kids happily get along and do not develop animosity and JEALOUSY when they have good, adequate and correct parenting - which is very rare, IMO.
Bad and inadequate parenting generally allows or causes the oldest kid to resent the next kid even though the oldest on knows that their parents, not the new kid, are hurting them (the oldest kid) but, since the oldest kid does not dare confront their own beloved and NEEDED parents, they turn their RAGE onto an easier and safer target - the new baby (you) who has never hurt them in any way! So your sister has most likely HATED you since the day your were born and INVADED her perfect life.
On the other hand, you most likely loved and respected your older sister all along because you've never had a reason to fear or hate her even though you may have fought back with her but never realizing that she absolutely hates your guts just for you being BORN!
If she stood up to the other girl's verbal abuse it was most likely because it has always been her DUTY, as the oldest kid, to defend and protect what is hers. She didn't do it for you. She did it as part of what is expected of the oldest child. SHE DID IT FOR HER PARENTS - NOT YOU!