OP,
It sounds like YOU would try to oblige...
But, your husband does not wish to oblige.
It sounds like your husband is the real sticking point.
It sounds like your husband is the one who is not wanting to work anything out...
If that is true... then... That just shows that he (as well as your brother and SIL) are showing NO respect for you...
If this is the case, then he is putting YOU in the middle...
He is asking YOU to forego this vacation with your family... and without a compelling reason... Only his preferences...
I have read enough here to make my very specific suggestion...
Tell your husband that you ARE going the vacation... It is your family... you wish to go with them... There is no compelling reason ( i.e. financial.. work obligations, the kids school obligations, etc... ) that you should NOT go.
Tell him that you will be going...
Tell him that you will be very happy for him and his son to take off during the week and do things to spend time together...
Tell him that you will be very happy to leave a couple of days early to avoid the whole religious/foods issue. (Or simply go out on your own, and eat at restaurants on those days...)
Tell him that you are compromising here, but that you will be going...
Seriously, at this point I would consider, taking the kids for the first 5 days, and leave him behind if he is actually being that obstinate.
Clearly, there are issues here that are bigger than the dietary restrictions... It sounds like even if SIL were to bend about making the entire vacation home Kosher, the OP's husband still does not really want to go... I am not sure that if the SIL were to bend, that this would change the OP's husband's mind.
PS: To everybody here...
It is NOT that easy to have a nutritious and satisfiying diet that is both kosher and vegan... This is NOT a 'small deal'...
It is also NOT okay for anyone to enforce their religious observances on others... Just because 'grandma' in the example above might go to mass while on vacation.. while on vacation with extended friends and family it would NOT be okay for her to expect everyone else to go to mass with her.. or to give up one of their vacations days, sitting around waiting on her to complete her religious observances...
The bottom line... The SIL is in the wrong.... and It sounds like the OP's husband is in the wrong... And they are both putting her in a position that really really sucks...
OP: I hope that you can speak with your husband and get this all worked out....