So sorry for your loss.
Here is what I would do ...
1) Take a look at the number of infections in your area and/or the areas you're travelling through to get to the funeral:
https://www.bbc.co.uk/news/uk-51768274
This will give you some idea of your chances of contracting Covid-19. Bear in mind that the true numbers are higher, but you'll get an idea of the order of magnitude. London is a particular hotspot that is probably best avoided.
2) Do you have any underlying health conditions?
3) Do you have anybody who depends on you?
4) If as a result of going you were to catch Covid-19 and ultimately die, in that period when you had the disease and before you died, what would you be thinking to yourself?
If you think on the above questions and decide to go ...
* Get there avoiding contact with as many people as possible - ideally drive yourself in your own car
* Avoid contact with as many people as you can at the funeral. Especially the funeral directors and others who come into regular contact with lots of people
* Listen for people with a persistent cough and definitely steer well clear of them.
* Don't go to the wake. Get out of there as soon as possible.
If you decide not to go, gently see if you can get somebody to video part or all of it for you, or at least get you a copy of the Order of Service - a young family member can probably video it on their phone for you. Also send a personal letter to other close family. Agree to have a good party in memory of your sister when all this is over.