The question of whether it's appropriate to let your boyfriend spend the night when you have a younger sibling in the same household can be subjective and depend on various factors such as cultural norms, family values, and individual beliefs. Different families have different boundaries and rules when it comes to cohabitation and overnight guests.
Your parents may be concerned about your younger sister being exposed to situations or discussions that they consider inappropriate for her age. At 15, she might be more curious and impressionable, and they may want to protect her from certain aspects of adult relationships. It's not necessarily about your boyfriend being a negative influence, but rather a general concern about maintaining a certain level of privacy and age-appropriate exposure in the household.
Having open communication with your parents about their concerns and your perspective can help bridge the gap. You might want to discuss why you feel it's okay for your boyfriend to stay over and how you can address any potential concerns your parents may have.
Ultimately, it's essential to respect your parents' rules and decisions while also expressing your own feelings and needs. Finding a compromise that respects everyone's boundaries and values is crucial for maintaining a healthy and harmonious family dynamic.