I figured I'd post an update, although not much has happened. I have seen them both at work, as usual, and they even came over to my house on lunch break to eat. I was able to escalate into a little more touching. (She asked me to massage her back) However, it is very difficult to get any alone time, as the sisters stick right next to each other. As such, I figured the best thing would be to plan an event with just me and the target.
As far as plans go, all three of us had plans to hang out and then go eat today. After work, they told me they had to go change and would be back. After they got home I got a message saying that they couldn't make it in time. I replied that we weren't eating until 12, it was a little after 9 at the time, and asked if they couldn't make that. The target replied that actually her and her sister were having a disagreement, but not to let the other sister know she told me.
I then asked what the target had planned the next day, which is our day off work. She told me nothing, so I told her to join me, by herself, at the zoo, with humor and an inside joke included in the message. She text me a lol and told me I should go with my other friends so they don't get "jelly". Then she said "I'll stop texting you so I don't take over your day lol I hope you have a good one!" I ignored it and teased her about being afraid of the zoo, because she had cancelled a trip to the zoo before. She replied that she loved animals and sent me a picture of her at the zoo from the past. I offered lighter plans instead and received a "Eeeeeeeeeee" reply. I didn't know what she was meaning by that, so I messaged back "Eeeeeeeeeee?" It then went on a couple of more messages and I text her "What was the tone of the Eeeeeeeee?

You sure are being complicated

". She didn't respond.
To summarize, the recent correspondence doesn't feel right. I understand she could have something going on and after typing this I feel much better actually. I'm mainly just using this post as a way to get my mind off of it and not overreact. Any thoughts? How about suggestions on getting her alone to progress further? I see her all the time at work and I feel like I'm taking too long to progress, but I'm not finding any chances.