A granddaughter-in-law's birthday is a special occasion that should be celebrated with love and appreciation. As a grandparent-in-law, you have the opportunity to show your granddaughter-in-law just how much she means to you and to the entire family. One way to celebrate your granddaughter-in-law's birthday is by sending her a heartfelt card or message. Let her know how much you care about her and how grateful you are to have her as a part of your family. You could also consider sending her a small gift or treating her to a special activity or outing, such as a nice meal or a day at the spa.