Time for Sully to inject some reality here. You have found yourself in a very complicated quagmire of a love triangle. You want one, you're with the other, she's with someone else, and you wanna make the switch without anyone getting their feelings hurt except Dan, right? No fucking way it happens like that.
#1. You're not the kinda guy that could pull off that switcheroo in the first place. I know this, because the kind of guy that could pull this off wouldn't be on here looking for advice on how to make this happen. He would already have pulled it off and be breaking Molly's back. If you need advice on how to pull this off, you have neither the necessary game nor the emotional detachment to make this happen. In short, if you coulda, you already woulda.
#2. This can't be done without lots of people's feelings getting hurt. Any chick that would go for some guy that her sister had just been dating isn't the kinda girl you really wanna be involved with long-term anyway. That's poor white redneck meth head trailer trash shit right there.
#3. The kind of guy that could pull this off wouldn't care about hurting the feelings of anyone involved. You can't make an omelette without breaking eggs, and u can't switch sisters without risking tearing apart a family. And if you had the emotional detachment to make this happen, you'd be the biggest douche bag in this situation, not Dan. Don't be that guy.
Walk away from the whole situation entirely. Give it some time, date other chicks and later, if the opportunity presents itself, take a run at Molly. But don't be surprised if it doesn't work out then either. Blood is thicker, bro. There's 3 billion chicks out there, I guarantee you can find another one that you're perfectly compatible with, if you're willing to look.