Except in infancy, Urinary Tract Infections (UTIs) are much more common in girls than in boys. This is because girls have a shorter urethra than boys and it is easier for the bacteria from the outside to move up the urethra and into the bladder. Ordinarily there should not be bacteria in the bladder.
There are abnormalities that some children may have in their kidneys, ureters, bladder, or urethra that make them more likely to get a urinary tract infection. The most common abnormality is called "vesico-ureteral reflux." This term describes a situation where some of the urine in the bladder goes back towards the kidney when a child urinates, rather than having all of the urine go through the urethra and out of the body. The way that this is diagnosed is by getting a special type of x-ray that looks at the bladder and kidneys while the child is urinating.
This type of reflux is not uncommon and runs in families. Thus if one child in the family has it, the other children should be evaluated for it. This refluxing of urine up to the kidneys contributes to more frequent and serious urinary tract infections, and if left untreated, can lead to scarring of the kidneys. Fortunately, most children who have reflux have only mild amounts of it, and they outgrow the reflux as they get older (the reflux stops happening). While they still have the reflux however, they are at much higher risk of getting a urine infection, and because of this higher risk, children who have reflux are put on low doses of antibiotics to take daily, to prevent them from getting an infection..
Based on what you describe, with both of your daughters having had more than one urine infection, I think they should be evaluated. You should talk with your physician about which tests would be the best to look at this problem. Since urine infections are so common, most children with a single urine infection do not need to have these studies done. However, children who have a UTI while less than 12 months of age, or girls who have repeated UTIs, should be evaluated.