If you don't make a Will
Your estate could pass to your Mother-in-Law
Whilst you may be happy that your Mother-in-Law would inherit your estate on your death, it may well not be what you would want.
How can this happen?
Well let's assume that you and your spouse / civil partner die together without Wills and that there are no surviving children from the relationship. In law it is not possible that two people die at exactly the same time. If there is no evidence to the contrary, the elder is always assumed to have predeceased the younger.
All joint assets that you own will pass to the the younger person. Intestacy rules are then applied and HEY PRESTO - mother-in-law inherits your estate, with your family getting none of the joint assets.
This is a good reason to make a Will in contemplation of marriage. If any accident should befall you on the honeymoon you can ensure that your chosen relatives / friends or charities will inherit your estate.