@jamilfamily, Looks like you only link to another person in the tree when you add a Partner (there is "Partner with person already on tree" button).
So, First you will have to add brother of your Grandma, then his son/daughter (your sister is law's parent), then their daughter (your sister-in-law). Now, when you add partner for your brother (click "Add partner/ex"), click "Partner with person already on tree" button. It will give you list of all person in your tree. Select the name of your sister-in-law.