Objectively, it's a tough call.
For me, with everything else being equal, I think a wedding is more important than a funeral. The bride and groom will remember their day for a long time, it is a happy occasion, the start of a new chapter in life, where you celebrate with them. There isn't really a way to celebrate a wedding after the fact, unless they have another party.
Whereas with a funeral is different - in my view, they are very much for the living, the person who has passed won't know or care who is there and there are many other, more personal ways they can be honoured and mourned.
BUT ultimately it would depend on the relationships.
For you:
If you were close to your dad and personally feel the need to go and say goodbye and / or you need to be there for your mom, siblings to grandparents / and or have a religious urging to go, then you should absolutely go.
If you weren't close to your dad and don't have family to support and you are only going because you think you should, or will be thought badly of if you don't, then consider what your relationship is with your SIL - are you close to them and likely to remain so?
If that's the case, then go to the wedding and not the funeral.
For DH:
Assuming that your DH has a decent relationship with his sister, then he should absolutely go to the wedding (unless there is some extreme backstory, where he was really close to your dad but has been low contact with his family for years.
For the children:
In terms of the children, how old are they? Did they know and have a relationship with your dad? Is it closer than the relationship with their aunt? If they are old enough to state a preference, what do they want to do?
If they are young and don't have a preference either way, they will have much more fun at the wedding and it will give you the headspace to deal with your grief.