If you do decide to email her, draft the message first and send it the next day, You may find a sleep will encourage you to reword it.

My personal advice is to keep the email short and sweet. Do not mention her relationship with her DSD or what you think of her as a mother. That is too personal and will only make things difficult and possibly put your DSD in a difficult spot between the two of you. A simple "I really feel I have tried to keep things civilised between us, but I feel you are not meeting me halfway, so I think it best if you only deal with DP and DSD from now on.

Good luck with this. It is so hard to have this conversation with someone but you will feel better afterwards

T
xx