hi
I was shocked by this letter,but not surprised. this is the way of the more fanatics JW, even if they are not all like this.
You could choose to ignore this letter , but this would be giving them the impression that you are wiling to cut ties with them, just like it is written in some Watchtower magazines. instead , as many people here told, you could Reply back with a gentle letter, saying that:
1) this décision is very sad and shows no love at all, given the fact that it is not a proof of love to disassociate and not even wanting to see people from family just because they choose to think differently. Ask her how she would react if situation were reversed, and if a catholic or a muslim people chose to disassociate from a member of family because he chose to become a JW...then put the extract saying that No one should be forced to worship in a way that he finds unacceptable or be made to choose between his beliefs and his family." (Awake! 2009 July p.29).
2) you are not willing to be as robotic as her, so you Will respect her décision and Will not be imposing your presence to her at any family event, or any other occasion, if it is zuch a torture for them to see you,but you Will never stop to love them so if one day they change their mine they Will always be welcome .also you wil be there for them if they need you for anything, they just have to give you a call.
I hope you and your family Will be ok...