Welcome to the forums!
You have certainly come to the right place as you will find all you need to know on budgie care here.
The information on the several articles and sticky threads will be extremely useful and of benefit to you and your budgies.
I agree that you have two young females there. Gender is not based from the size of a budgie. The size is something they will inherit from the parents, smaller parents will have smaller offspring, diet also plays an important role in the healthy development of the chicks.
Besides following quarantine procedures for your new budgie Sherlock, you will also have to take precautions in order to discourage future breeding behaviour that can lead to unwanted egg laying. This is done by not providing hiding areas nor anything that they can use as a nest, you will find the detailed information on this link: When We Don't Want Eggs!
If in the future, you are considering to house your sister's male budgie with your girls, this is also not advisable, not only will one of the budgies be necessarily left out, but also the chances for fights to happen are higher: Budgie Ownership: differences and dynamics between flocks
Best of luck with everything!