First of all I'd like to congratulate you on taking in these chicks, looking at the pics I see good and full crops and you are doing an excellent job so far!
They are very young and while the colour on their ceres will suffer a lot of changes as they grow, I do agree with your guesses on gender.
Yellow is definitely a girl and Blue is possibly a boy.
The chicks are at a good age for your to introduce other types of food besides the baby budgie formula. You can start by offering them egg food. You can buy egg food at any specialized pet store or you can make your own by hard boiling an egg and mash it all up, you can also mix in finely chopped up veggies and sprinkle some easy to dehusk seed.
My chicks love and always pick the Niger seed first, so you can put place some on the egg food. Other easy seeds for them to crack open and eat are millet spray, canary seed and flax seed.
For now it's best to keep the siblings together because it is very helpful during the weaning process, the siblings will encourage each other into eating, once one starts to pick on the food, the other will follow suit.
As far as taming goes, you can use the budgie who is most bonded to you, in this case Blue to teach Yellow that you are a friend and part of the flock. They are still very young, so establishing the foundations of trust for a good bond will come naturally with time.
It's perfectly possible to house two siblings of different genders together without the threat of possible matings. As long as you don't give them any nesting sites nor provide them with hiding areas, then chances are high they won't think about raising a family and you shouldn't have problems with unwanted matings resulting in eggs.
It's also okay for them to be bonded to each other, to preen each other and for some food to be exchanged between them.
Best of luck in finishing raising your chicks!