You can't prevent any upset happening at all, as whenever a partnership breaks down there will be upset, but you can smooth it for the children. His family will have to face up to his drinking as whilst you have stayed with him they have probably been in a comfy position of denial & 'well its not as bad as all that' which will change once you have gone. At the moment you are taking the brunt of living with an addict, once you have gone then he may expect them to fill that role & they will see what you have had to put up with. Tell your family as well, you have nothing to be ashamed of.
The girls at their ages may know more of what's going on than you think & remember that no child wants to be the cause of a parent staying in an unhappy relationship. Just keep it calm & keep the main crux of the matter as that you & partner will be happier living apart, no need to mention the women, although my DC are still quite small so I'm not sure how much a teen would understand of alcoholism. Feel your way through with sensitivity & it should be fine.
If you are worried about the family reaction then would it possible to contact them after you have told partner he needs to leave? So they have your explanation first before he gets in there with his half truths & blaming it all on you?
See a solicitor or the CAB for advice about joint assets & how to protect your things.
The scariest thing is actually doing it, once its done & he's gone then your day to day life will be so much easier. There may also be a huge blissful feeling of relief which can get you through the first few days.
One word of caution though, after the initial aftershock he may get very spiteful & do things that you think he's not capable of, so keep a very close eye on any joint monies you have & think around what relationship you would like to have with the 11 year old, if any. He may try to hurt you through any means he can, but just ignore & see it for what it is. You are making the right decision & it takes courage to look your relationship straight in the face & see what's wrong with it. Well done, & it will get better 