Neither

After all that has happened, after all of the mischief, all of the laughter, all of the crying, all of the snark, all of it, one thing has become absolutely certain. You love Vanessa and Olivia dearly... just not in that way.
You look down with a frown covering your face.
"I'm so sorry Olivia." you tearfully say.
"It's okay..." she trails off: "I'm glad you and my sister found happiness together."
"No... I mean, I don't have those feelings for either of you."
Olivia's crying just suddenly stops.
"Oh..." is all she can manage to say.

"I'm so sorry Ollie. I didn't mean to egg you and Vee on by my inaction. I got so confused, and I couldn't find myself. I am so so so sorry."

Olivia looks at you. You can't read her face.

"It's funny," she starts: "It didn't even occur to me that this was a possibility. But no, you're right. It's all okay."
She doesn't sound okay.
"I wish it was different." you respond solemnly.
"Me too." she agrees.

She wipes her tears away.
"I told you at the campus that no matter what you chose to do that we would be a family. And I still believe that." she says starting to cheer up.
"Me too." you agree.

Vanessa's reaction mimics Olivia's pretty accurately. However, with Olivia's help, it doesn't take too long to comfort her as well, and after a bit of tearful talking, you are all watching a comedy movie to cheer yourselves up after the whole ordeal.

The following weeks are awkward, but not excruciatingly unpleasant. Eventually, you all move on. Months go by, and the decision of letting the sisters move in only seems to become better and better. Olivia decides to start college. She finds herself a girlfriend and after a couple of years finding themselves (and each other) they move in together.
You help Vanessa find a retail job and a small side-thing with her art. You tell her if she or Olivia ever gets into financial trouble they just need to ask. After acquiring a stable enough income, she eventually moves out as well, tearfully thanking you for everything.
While love never entered into it, you can't help but feel attached to the girls. You have become their unofficial third sister, and you still see each other on a very regular basis.
As Olivia marries her college girlfriend, you and Vanessa are the only ones to represent Ollie's side of the family. You are now truly their family.
And as you sit there in the church hall wondering what the future has in store for you, Vee, and Ollie, you know that whatever bullshit life throws at you, you will support each other till the bitter end, because that is what family does.

The End