What should I do about my shyness in speaking with girls, especially with my sisters' desire for me to be more bold?

There is no good in sitting with girls, and modesty and turning one's heart away from intermingling with women are a blessing and a good. The Prophet, peace and blessings be upon him, warned against the trial of women, saying: "I have not left behind me any trial more harmful to men than women," and "Beware of the world, and beware of women, for the first trial of the Children of Israel was concerning women," and "No man should be alone with a woman who is not lawful for him, for indeed the third of them is Satan, except if there is a mahram (definition)." It is not permissible to be in seclusion with a non-mahram woman unless her mahram is present, and your sister is not a mahram. It is also not permissible to look at women, for Allah Almighty says: "Tell the believing men to reduce [some] of their vision and guard their private parts." Your shyness and distance from women are good and a blessing, and whoever protects his limbs from the unlawful, Allah will preserve them for him in the lawful.