So sorry to hear about your lose. I know that at this moment it seems just another insurmountable hurdle that you must climb. But the number one thing to do is to keep things in perspective and to handle them one thing at a time. When you try to think about everything that you have to do all at once, it makes you stressed and confused. Your FI is very supportive, so you are not in this alone. First lock down your travel arrangements to the funeral. Then sort out exactly what you have left to do and what you can do on the bus ride to the funeral. As you mentioned, let your FI take care of the things that you cannot take care of while you are gone. Try not to worry about things. Go and grieve for your aunt, be with your family. It is healthy and it is what you need. You don't want to feel guilty that you were at your aunt's funeral but doing wedding planning every chance you got.
Most importantly, breath and take some time for yourself anytime you get it. It is going to be rough, you are going to be tired and you are going to be stressed. But to lessen all of that just remember to keep breathing, stop and enjoy a moment or two and just like you take each step one in front of another, one at a time, that is how you handle things that will be faced with in the next fews days.
I hope this helps. Good luck! Sorry about your aunt.