The STBX-IL has assumed OP will go to the wedding, 4+ hours away, spend 2 days there and take unpaid leave, with the kennelling
Perhaps not altogether unreasonable, it is afterall an invitation... but just that.
the Extremely Unreasonable thing the SIL2B has done is to make the assumption that this person will come to the wedding and MISS OUT on the actual proceedings to look after the B&G kids and potentially the DC of others.
She is being used as hired help, apart from the fact that she's not actually being hired or paid to do so.
Worse than this is rather than question her own decision to make these assumptions or, heaven forfend actually ASK the OP if she could provide all childcare all day at extreme cost loss to her, she has the bare arsed cheek to kick off at her and verbally abuse her into the bargain.
I'd say that this is a flaming liberty, but it looks more like an informal slave deal so I won't use a word relating to freedom.
OP, this is outrageous and I would strongly suggest your tack from this point on is to say that you won't discuss it anymore, that not only are you unable to attend, you are now UNWILLING to attend.
I would actually point out to her that even if you were able to get the time off, she is not worth the effort it will take to get there to be at her beck and call even if they were to cover your costs and losses for the day, and actually pay you to be there.
there is room for someone to be outraged here OP, and it ain't Gluezilla.
In terms of your family applying pressure, just repeat the We can't get the time off line, and refuse to be pressured any further.