Are they in love????

well my sister and my cousin are close to age but not really shes 18 and hes 23 well he goes everywere with her and if he needs to go to some place like to dinner with his friends he will invite my sister and will like never want to leave the house with out her like and if we go somewhere with my moms family he has to go and she always asks if he can go or come she never wants to go anywhere with out him when i invite her she claims she doesn't like going to wre ever i go they stay home alone and like if shes depressed she will run to him and tell him everything she used to cry on his lap their always in his room alone with the dooor closed one day i told them i was going to take a shower they would rush me into doing it like wanting to get rid of me one day i even caught them i guess doing something they werent suppose to and when i went in the room they acted alll normal every time i walk into the room thay change thw subject and one time he woke me up thinking i was my sister but he touched my thigh like in a sexual way then said to wake up then he said sorry i though you were your sister and everybody in my moms family says that maybe they have had sex or are going out also his dad is a pervert so maybe he is to tell me am i over reacting or what do you think they have a thing going on or what i need to know thank you