shiny dream is right that usually , its our thoughts that stimulate us, but nerve endings in nipple can trigger sexual responce in some cases apparently, even if we arent thinking about it,
i agree normally we will surpress feeling if its family , but sometimes arousal happen any way.
like i read once of lady who was raped, but she said rapist was gentle with her, and she reached her sexual climax.
she certainly didnt want that to happen, nor was she a willing partner, but the stimulation from him triggered her physical responses ,and she was unable to do anything about it . so she had an orgasm, but it did not mean she wanted that.
it just how her body responded to the stimulation, probably of her clitoris in this case.
so nipple stimulation can happen too, even if we are not desiring it .