My Late Sister’s Husband Wants My Hand In Marriage

Hi friends, I need your readers urgent advice. Please this is important to me. Thank you.I am a 27 years old lady who has been living with my elder sister’s family for over ten years. Unfortunately I lost my sister two years ago and have been of help to her widower by taking care of my three nieces. However the situation is already out of hand as my sister’s widower is getting used to my company. He has been taking me out for romantic dinners and outings and eventually proposed marriage. He often tells me he sees my sister in me, and loves the way I care and show love to my nieces which I think it’s natural to show love to relatives notwithstanding if they are dead or alive.He needs my positive answer to inform my parents back in the village about his intentions. I’m so confused and cannot come to a decision on this whole issue at hand. Is it a taboo? .Don’t even know what next step to take. Dear readers I need your candid opinion on this. Thanks