Yes, this sounds like abuse. Being your cousin was very young when she started masturbating (I mean, she probably started before that) and she had no regards for privacy or you being just a little kid, she might have been a victim of abuse herself. Any sexual encounter involving unwilling participant is sexual abuse. I don't know how old you are now, but you should mention this to your mother, confiding in someone always helps. You should also visit a therapist, having recurring dreams about a past event means unresolved issue/trauma. In the end of the day you might wanna talk to your cousin (if she is still in your life) tell her you remember this and ask her if she was ever touched or otherwise molested in her childhood. If she apologizes for what she had done to you it might be a first step to putting this to rest.