Hopefully it was just a figure of speech.

Maybe OP wasn't there for her sister? I mean, siblings are either really close, or really distant. At least for me, growing up, my sister and I hated each other. But now, we act like family.

Regardless, it was still a bit inconsiderate. If my sister said that to a large group of people in front of me I would feel extremely shitty.

I've grown up to be very distant with my older sister.
I have a valid reason for it, and my sister did something to end up in that position. It could be the same with OP.
Either she wasn't there for her sister and she did something to be disregarded like that. Or maybe her sister doesn't know/appreciate what a great sister OP might be. I would hope it's the second one, because it feels terrible to be distant from siblings.

Maybe you're adopted o_O

She's still her sister...

And maybe it was the worst decision your mother ever made!!

Still her sister, genius!

Who knows maybe there's a part she is going to surprise you with!..probably not but you can hope!

Maybe she says it for dramatic effect.

Wow. That's pretty harsh. Hopefully it was a mistake.

If it isn't, let your sister know that it bothers you. Hopefully, like #5 said, it was a mistake.

Harsh, but maybe she's ignorant of what "sister I never had" means or just got confused between that and "like a sister."

Dont worry OP, my brother pretended I didn't exist for a month... Im sure it was just a figure of speech

Maybe she wanted another sister

Ehhh.. probably not considering the way the OP put it.

Well I obviously don't know you or your family, but if that's in her speech, maybe you should've been more supportive of her?

Don't worry, blood is thicker than water. You'll always be sisters no matter what. Maybe talk to her about it?

Fun fact, that full phrase is "the blood of the covenant is thicker than the water of the womb," meaning sometimes you have to choose friends over family... Which OP's sister appears to have done.