I am sorry to note that your mother has what you think is an umbilical hernia. If the swelling presents when your mother strains or stands up and reduces (disappears) on lying down then it is most probably an umbilical hernia. If she develops pain in the swelling or if it does not reduce on lying down, then she should see a doctor immediately as the hernia could have obstructed.
Operation per se is not a very major one. However, occasionally complications such as wound infection and very rarely mesh rejection (when used) may occur. The risk of any operation also depends on the general condition, whether she has co-morbid conditions such as hypertension, diabetes, coronary artery disease, renal or pulmonary disease, all of which are adverse factors and increase the risk of an operation.
This is not a hereditary condition; your surgeon will be able to guide you in an appropriate manner after he / she seen your mother.