K.C.
Really, for a wake, your just being there to provide emotional support to your loved one is usually enough. Depending on the faith of the deceased and/or their family you could bring a mass card. Another thought would be to bring casserole or something for your brother in law so he and/or his wife/family don't have to worry about food in these difficult days. If you do that, pack it in a cooler bag with ice packs. Include a card with instructions for heating and tell them to keep the bag and dish so they don't have to worry about returning it.
You could also make a donation in the name of the deceased to her favorite charity or organization or if she died from an illness such as cancer, donate to a cancer charity.
These are all just things that come to mind but you just being there to lend support really will be enough. I don't know if your brother in law has kids but maybe offer to babysit them should the family need to run errands or just need to not worry about kids. Whatever you do will be appreciated by the grieving family.