Intermingling between non-Mahram relatives

Q: My father married a woman who bore four daughters. One of them is married and the others are all grown. He also had four sons by this women, the eldest of whom is a foster brother to my cousin. Thus, the four girls are foster sisters to my cousin, or so they believe. My father then married my mother who gave birth to me and my sister and later died (may Allah have mercy on her). Afterwards, my father married a third woman who bore him a daughter who got married to my cousin. My cousin lives in Abha, whereas my father and my sisters live in Jizan. After my cousin's marriage to my sister, (Part No. 17; Page No. 77) my other sister went to live with them from the age of seven until she became thirteen when she returned to her father's house. Now she deals with my cousin as if he were her father. She travels with him at night from Jizan to Abha without a Mahram (spouse or permanently unmarriageable relative), she exposes her legs in front of him if she has a rash, and kisses him in front of us as a daughter does her father. This girl is now seventeen years old, and my cousin is thirty six years old. The problem is that my cousin lives with us in the same house in Jizan. He jokes with my sisters in front of us and our father, he might sit in Khulwah (unchaparoned) with them, and travel with them without a Mahram to Jeddah or Abha. If anyone of them becomes ill, he takes her in his arms to the car and takes her to the hospital.I do not believe that he is a Mahram for all my sisters. I told my father that this is wrong, and he admitted it was, but claims these are the customs just as neighbors kiss each other and a cousin is considered a Mahram. In addition to my cousin, most of our adult neighbors enter our house without taking permission and meet my sisters without my father's objection. My father does not accept any discussion on this issue. He tells us that they are his daughters, and none of us has authority over them (Part No. 17; Page No. 78) as long as he lives. My father, may Allah reward him and forgive him, also does not agree with me on this jealousy. If he finds me with one of my sisters in her room discussing a private matter, he gets angry and says, It is impermissible for a man to sit in Khulwah with a woman, even if she is a Mahram for him. Please advise concerning the following situation: is it permissible for my cousin to joke with my sisters verbally and physically and sit in Khulwah with them as long as my father does not mind? Is it permissible for neighbors and non-Mahram relatives to enter the house without taking permission and meet my sisters? Is it permissible for me and my brothers to sit in Khulwah with our sisters or travel with them without another Mahram? Are my objections to the situation at home valid or not? What should I do so as not to be blameworthy for witnessing these situations I have described? What is the solution? May Allah guide you to what He loves and is pleasing to Him.