is it sin to cast spells without consent. because it is not possible to cast love spell with consent, on a person you love most but does’t get the same in return.Does it harm.

Wicca does not really hold the concept of “sin;” that is really a Christian concept.  But let’s look at your question in detail.

I think that what you’re really asking is if it is wrong to cast a “love spell” on someone without their consent.  If you did not already know the answer to this question in your heart, you would not need to ask.  Wiccans hold that white magick requires consent.  If you cast without that consent, you are straying into the categories of grey and black magick, and opening yourself up to a lot of harm in the process. 

You’re right; it is not possible to cast a love spell on someone who doesn’t love you in return, and expect consent.  What you need to do is change your focus.  I know how difficult it is to love, and to have that love rejected.  However hard it might be, it is triply hard to receive the results of that spell back three-fold.  You should know the Wiccan Rede and the Rule of Three.  The Rede says, “If it harms none, do what  you will.”  The Rede states, “Whatever good that you do, three times returns to you.  Whatever harm that you do, three times returns too.”  I don’t know about you, but I would most certainly not want someone casting a spell which directly interfered with my volition, my personal choice of who I will love.  I don’t think you’d like that, either.  Don’t cast upon this person, please.

Instead, I’d like you to recognize that what we think we want is not always what is best for us, not always what we need.  We sometimes squall like a toddler denied a snack, when the Universe, like a wise parent, is telling us, “Child, shhhh!  Come and see what wonderful things I have for you!”

I would suggest that, instead of casting on this person who does not love you, that you cleanse yourself of negativity.  Once you are focused outside of yourself and what you want, why don’t you try casting for attracting the love of your life?  The Universe knows the person who is completely right for you, but you will never find that person until you release what you need to leave behind.

Blessed be.