The most complicated situations are most of the time resolved with the easiest tasks. The real problem is how hard the easy task is for your emotions and mental state.

Advice is easy to give. Leave him. There, easy.

Advice isn't what you need. You know what to do, deep down. What you need is to get your head in the right place, so you can do that "easy" thing with an "easy" outlook of it, not a complicated one.

You don't want to make that leap because you've attached the hurt you feel right now to the unknown future that you imagine when you think about confronting them, and ultimately leaving him.

The hard cold truth is that, if you wait, the pain will buckle you down. It will not be easier to deal with whatsoever. This could potentially cause you to buckle down for good. Get used to it. Then it becomes a whole different story. One where you are never happy, and all the power and freedom you have now goes flying out the door.

The love he has for you is outweighed by his lust. Sexual temptation is something anyone with enough love for another can overcome, if they really cared enough.

The exceptions to this, I see as thrown out the window considering your own sister is on the other side of the affair. This means it's calculated (obviously with the texts and all), and unfortunately, since she's your sister, shows how quick he is to do such things, being she is "easy access". As you've said, you're close.

If it's this easy for him, to go straight for the closest person to you, then you don't stand a chance against the whole breathing world out there full of women.

Again, you know what to do. You just want the courage to do it. Find that courage (my advice is not at the bottom of a bottle. Keep your head on straight) and do the "easy" thing. Get your shit in order, and leave him.