When WikiTree uses the word "merge", it is different from what you want to do. A merge is done when we discover that there are 2 profiles for the same person because this is one single tree for all of humanity, so there needs to be only 1 profile per person.
You have just joined (congrats and welcome!) and I believe what you want to do is to add a profile for your mother. On your profile, click the Edit tab and look on the right side, where there is a place for lists of father, mother, siblings, spouse(s), and children. Click one of the "Add" links there to add a new family member and you will then create a new profile for that family member.