This is not about 'a cup of tea'.
No, it is a mere detail but those little details make up the larger problem which then becomes insurmountable.
Are you, perhaps, keeping score rather than just freely being generous?
In one way or another, several posters have wondered this (including myself) so maybe, without meaning to, you are giving your son-in-law and daughter a similar impression. Whenever there is a family breakdown, neither side is usually blameless and I think it is really important to look at where we might have contributed to the situation and take responsibility to that. Just as an example, with the cup of tea, your son-in-law might have been thoughtless but your husband could have just said something like, "2 sugars in mine please," with a smile just to remind SIL that he needed a drink too. Maybe after that, your SIL would have remembered your husband when he made the tea.
I can see why you are unhappy with the way your daughter has acted with Social Media and it seems to me that this is the way the next generation deal with registering their hurt or anger. It is childish and as such, I try not to let it get under my skin too much. Just bide your time until tempers have soothed a little and then when you get an opportunity as if she would like to discuss what has gone wrong. Before you do that, ask yourself what you are trying to achieve. Are you trying to resolve the situation or point scoring to try to get validation for your feelings. It will only work if you are striving for resolution.