A mother-in-law can welcome her future daughter-in-law to her family by giving her a gift. The gift does not have to be expensive or extravagant. It simply needs to show the recipient that the mother-in-law is excited to welcome her into the family.
One potential gift idea is a homemade recipe book. The mother-in-law can gather all of her favorite recipes, type them up and bind them together in a book to present to the daughter-in-law. This shows the future daughter-in-law that she is now a part of the family and needs to know the family's favorite recipes.
A mother-in-law can welcome her future daughter-in-law by spending quality time with her. She can invite her to lunch, a shopping trip, a movie or a spa day. This shows that the mother-in-law is interested in getting to know her future daughter-in-law and wants to develop a relationship with her.