If they were the same age and wasted their time by playing football together or stealing apples from the nearest garden or painting on walls. So, yes, "Valyukha" (Валюха) or "Val'ka" (Валька) like bro to bro. But taking into account the age difference (23-16) there is rather patronizing than just bro to bro friendship. If they were a real brother and a real sister then again he could call her "Valyukha" (Валюха) or "Val'ka" (Валька). But real brother and sister have another level of relationship than just friends.