If you truley believe that she is you twin flame, then this can simply be seen as one of the many obsticals to overcome in time. The believe of a twin soul follows the understanding that twins can meet in their lifetime but often they are not ready for their connection and their relationship may not last due to the emotional baggage. Others that have found each other also state that they had to endure much to get to each other, so in a sense her engagement to another person may mean that she isn't ready to be one with you yet (now or in this lifetime-perhaps another lifetime) OR it may be that your attraction to her is so intense you believe her to be your twin and in reality she isn't.
At any rate, love spells can cause disruption and in the end if you aren't meant to be together it won't matter how many spells you try. As difficult as it is, and I know love is a strange emotion that makes many feel and do irrational things, I would focus on not trying to cast a spell to make her love you but rather wait and see what unfolds on its own. The universe is so amazing if you just give it time, it may show you she is the one and it will work out, or you may find another that you connect with on an even higher level.
In all I wish you good luck and hope that this gives you some hope and comfort.