Firstly, I'm so sorry for your loss.
you are hurting and focussing on something that isn't all that important, the logistics.
Who is the other adult in the car? Are they close family? To put it bluntly, are they more important to you than your sister?
If you weren't giving them a lift, how much of an issue would your sister taking the baby be? Could a solution be contacting them and saying you are sorry, but there's no room in the car and they will have to make their own way there. Could they drive/get the train rather than your sister trying to travel with a baby when grieving her gran?
For 2 nights, I would say your sister can co-sleep, even if they don't normally, that would be the 'where to sleep' problem fixed.
You need to think past the funeral, you could do long term damage to your relationship with your sister if you don't help her now. As independent as you are, don't assume you'll never need her in the future. Family is important.