D.K.
The best way to handle someone filled so much full of pity for themselves and bitterness, is acknowledge you hear what she is saying, that it is her choice to have her point of view and feel the way she does. However, this isn't about her! It is about a choice your mother (who by the way is a grown woman) and you made together. It is not about what she is going to not have however it is about what your mother wants to do. She sounds like her life is very unhappy and she wants those around her to be unhappy too and tries to control people with mean words. Manipulation is dangerous territory and more often then not will end someone a very lonely life. You can explain to her that though her words hurt, you are going to look past them and if she wants to discuss her feelings like a grown up then you are willing to, however she is not to lay guilt on you for you having a life of your own, that is what people are suppose to do. Family is suppose to support that and not try and bring you down.
If you want, type a response, reread it a million times and save it somewhere on your computer. Then sit on it for five days. If at the end of those five days you feel still compelled to send it, do so. However reread it after your feelings have calmed down so to make sure you don't send something harsher then it has to be.
You don't have to get in a war of words, she is entitled to her opinion as you are entitled to your living the life you want. She has a choice, to be part of your life and happy for you or not. It is her choice. You can tell her you would love her to be part of your life however her negativity and guilt trips are not acceptable. You teach people how to treat you and she feels somehow her emotional manipulation is going to get her what she wants, it won't.
Taking the high road is a good feeling sometimes too.
Hang in there, you are very blessed to have a mom there to help you out, be there for her grandkids and have the good life you do. Your sister will understand that one day!
God Bless