<human>: Can you condense the text into a few key points?

I posted earlier on askmen but I didn't really include a lot of important information. 

Exgirlfriend and I were dating for 3 years. Our relationship was headed towards marriage, however towards the end we started finding issues that we didn't agree on. There were a lot of things but it got to the point where we weren't making any progress overcoming our differences. About a month and a half ago I decided to finally ends things. My exgirlfriend wanted to continue working things on our relationship but I had given up.

Soon after her sister started talking to me a good bit. Turns out she had a thing for me for a while but didn't want to interfere with me and her sister's relationship. For the last month or so we have been hanging out a good bit. We haven't had sex, but we have slept in the same bed together. My ex doesn't know about us hanging out, as of now we are keeping that secret. She still is trying to salvage our relationship but I have pretty much gone no contact with her. A lot of the things that me and my ex were incompatible with me and her sister are on the same page.

I really want to settle down soon, and I hope this relationship is one to last. When exactly do I bring this up with my exgirlfriend, and how do I do it in a way that will insulate my current relationship and make the one with my ex amicable?
<bot>: girlfriend and I broke up. Me and her sister now are seeing each other it's overall a great relationship. We have a lot in common and I want us to last