Well done on booking respite for her, I remember you mentioning it on your other thread.
It may well be best to forget about the holiday. Your mother now has to get used to a new environment at the care home, added to which your brother is very unwell, so things have changed since you booked it - and even then you didn't really think it was a good idea and only did it because your mother insisted. Your SIL now has other priorities, and I suspect your mother would cope badly with the challenges of travelling and it would be a lot less enjoyable than she expects. With this illness you have to constantly adapt to circumstances, and make decisions on her behalf - as you're doing with the care home.
It may well be best to forget about the holiday. Your mother now has to get used to a new environment at the care home, added to which your brother is very unwell, so things have changed since you booked it - and even then you didn't really think it was a good idea and only did it because your mother insisted. Your SIL now has other priorities, and I suspect your mother would cope badly with the challenges of travelling and it would be a lot less enjoyable than she expects. With this illness you have to constantly adapt to circumstances, and make decisions on her behalf - as you're doing with the care home.