Ex told me a week before Christmas that he was leaving. I'd caught him out at the beginning of November but begged him to stay, I was terrified of ripping the kids worlds apart. He spent 6 weeks trying to decide what to do, poor soul 
. In the meantime I became a total wreck. My weight crashed to 5.5 stone, I was ill with a chest infection that wouldn't shift and my gp wanted to admit me to hospital to get me away from the whole situation. I didn't tell a soul what was going on apart from my MIL. The children didn't know anything was going on. I wasn't allowed to mention his affair as that just gave him the proof he needed that I wouldn't be able to move on from it and he would swing from saying he would stay to deciding to leave.
When he finally made his mind up it was a relief almost. But he refused to go before Christmas was over and I had to decide whether to drop the bomb on everyone before it or after it. I chose the latter and it was utter hell. I had to listen whilst my dad repeated constantly what a wonderful husband I had as he cooked the Christmas dinner!
He told the children on the 27th and left that day. It got easier once he was out of the house, even though the kids were in pieces but I had to kick myself hard and be strong for them which helped me through it also.
Your sister sounds exactly what you need right now, someone to hold you together when you're unable to.
Be careful what you say to your ex, even giving him warning of what you're entitled to when he isn't already aware will help him prepare his finances etc. Let him find out in time from his own legal representation.
When the time comes, you will both have to fill out a Form E. This is your financial declaration where you will have to detail all income and expenditure (right down to the last penny so even things like toiletries and presents for the kids friend's birthdays etc) which will help you work out what you need versus what you're getting currently.
I don't think you're being naive but I do believe from my experience there's a definite element of wishful thinking. I remember my ex, just before he left when he still felt a tiny modicum of guilt, promising me he'd even set up a holiday fund for me and the kids so that they wouldn't have less of a standard of living when with me than they do with him. Needless to say, that hasn't happened. He isn't even paying maintenance and won't contribute to their uniforms this month in lieu of maintenance so I actually laugh when I think back to that earlier conversation!
I started divorce proceedings in March. He has caused delays all the way through but we got the decree Nisi at the beginning of July. I could, in theory, apply for the absolute now but I'm not going to until we've reached financial settlement. First court date could be as soon as November. If your ex plays ball then it can happen quicker than it has for me, my ex has refused to respond to most things and needed chasing up every time. I have also spent over £3,000 so far and I'm looking at around £25-28,000 if it goes to a 3rd hearing with barristers. Scary stuff.