Wow! Your SIL is pretty sick! I think you doing the right things. So you "cracked" under his enabling pressure, you're only human. I'm an alcoholic and I would have felt the same way on the boat! I think protecting your son is number one in all of this and if that means taking a separate car to family functions so be it!
ALANON will really help you. There is also a friends and family section on this site. Good for you on setting boundaries, your husband probably has no clue what those are right now but be easy on him as this is all he's known. Hopefully he will get some help too! Welcome!