Well, the whole idea behind a funeral is that it's a ceremonial way for family and friends to pay their last respects to the individual that has passed away. Also, it provides a gathering where the children and spouse of the individual who passed away can be offered condolences and emotional support too. It is not unusual for individuals to attend a funeral of someone they do not know simply to provide emotional support for a spouse or child of the deceased that they do know.

So beauflow, if you do choose to attend this funeral you don't have to stay very long and you don't have to "be" anything other than one who is paying their final respects and perhaps just offer your quiet condolences to your sister's husband and children if there are any of these survivors left behind in your sister's own family. The deceased person doesn't have to be a big part of your own life in order to pay your last respects and you don't have to feel guilty if this individual and you did not have a close relationship either because often people do end up distancing from their family and can lose touch and this is especially true in the present in that it's not unusual for a family to spread out and settle in different states. Long ago families tended to stay in one area, but that is no longer the case.