L.D.
First of all, I am very sorry to hear about your husband. :(
Second of all, I would absolutely talk to your MIL in as calm and non-accusing a fashion as you can. Tell yourself she is just oblivious to the fact she is doing this. I would tell her that you have noticed she is favoring your son over your daughter again and then give specific examples like the candies so that she has something specific to think about. Tell her that as your children are getting older, they are becoming aware of her favortism as well and that it is hurting your daughter's feelings. In all honesty, I'd even throw out there somehow that your husband would want to know both of his children are being loved equally by his mother/parents. Maybe knowing it is hurting your daughter and would be hurting her own son would be enough to make her give more thought to her behavior in the future.
Hugs,
L.