This sounds so stressful but you're going to be okay. I realize it all is so complicated because extended family became involved and everyone is talking and possibly fueling anger.

Try to keep it as simple as possible. How you next proceed in your relationship is between YOU and YOUR PARTNER and it's important to explore everything that happened with just him and a therapist. I realize everyone "feels" like they're deeply involved but what happens next is not up to anyone else but you two.

At some point, you and your partner can address this with everyone who was impacted, (like his sister, etc) and let them know you're working through this and need their positive support - for the sake of the kids! It sounds like you've already been accountable for your actions but it would help to acknowledge their feelings, how uncomfortable and sad it made them to witness this and most of all state your intentions if you decide to be committed to rebuilding trust. Your partner will need to "call off the dogs" if you both are committed to moving forward.

Take care and don't beat yourself up. When we are open to doing these types of things, we are trying to kill pain, trying to fill a void, and, as Bill said, there are certain things that make us more vulnerable to acting out. Explore all of it and be honest with yourself. You can do it!