I have researched BPD....does it really make one CRUEL? because I took that as a slap....it just ruined my whole day (if I am being overly sensitive, feel free to tell me

I think in some situations, a pwBPD can do cruel things. Doing something cruel doesn't specify the exact motive. Why they would do it, I don't know. I think they have their own emotional issues and project them. I also think they percieve themselves in victim positon and then lash out at whoever they perceive has hurt them, whether it's actually true or not. PwBPD tend to be impulsive. What I think is less likely is that they'd actually be thinking "how can I be mean to ________"? and plan it out. I don't think your BPD child may even be focused on how her behavior affects you, just her own thinking.
Family dysfunction can affect all family members who may behave in dysfunctional ways- not because they intend to necessarily but because this is what they perceive as the "normal" in their family. The pattern can pass down to generations that also replicate what they see as "normal".
What I see in your posts ( and keep in mind, we can't know all of any situation) is that the dysfunction goes beyond your daughter. There seems to be triangulation- daughter- your mother- you. ,and a lot of people communicating indirectly through triangles. Your mother appears to be in rescuer position with your D and also possibly with you. She's the go between. While this is better than no communication, it may also be perpetuating this situation. Your D doesn't have to communicate with you because, she can communicate through your mother.
Other family members also participate in the dysfunction. Your sister and neice as well.
This is hurtful to you. I have been on the receiving end of this kind of dynamic with my BPD mother and her FOO who aligned with her. It's a very hurtful situation. I won't ask you to not be sad about it. That's impossible to not be sad that it's going on.
However, I also think it's possible to not increase the personal suffering from this by - quietly- don't say anything to anyone about it- that will play into the drama- creating some distance from this for your own emotional well being. Your feeling hurt doesn't change your D, or anyone else in the family. It has no impact on their decisions. But it affects you, and that's the reason to try to diminish it if possible.
For me, it was to step away from the triangulation. Not reach out to a family member to discuss another family member or if they bring it up, to say as little as possible, change the subject. I also kept my own personal information to myself, knowing that whatever I said to one of them would be shared with the others. I did not anounce it, that would cause drama. I just did it.
You want to keep the communication doors open. Of course if something was important, you want to know about it. But you don't need to know who gets together when, what they say or don't say. FB posts don't mean a lot- people put up an image on FB that they want people to see. For all you know, that Mother's Day get together was full of drama that you didn't have to get involved with. BPD affects all relationships. It may appear they are getting along without you, but you don't know if it's true or not.
People feel most aligned when they have a common person to blame. It may be that once you step away from the triangle, they might focus on someone else.
I don't like the idea of unfriending family on FB. However, there is a way to hide your posts from them and also see less of theirs without unfriending them. They won't even notice. You can selectively hide your posts and show a few so they don't catch on. It just creates some emotional distance from them for you.
When you speak to your mother - don't ask about your D as much, maybe not even at all for a while. If she brings it up - keep your responses less emotional. If she asks how you are doing, tell her you are doing better- don't go into it with her. If your D wants to know your feelings, she can ask you herself.
It's hard to feel happy but I hope you give yourself permission to do so. It isn't all the time. There will be a song on the radio, or a picture or post, that brings up the sadness. These relationships are significant and the emptiness is a part of this. But you are worth having some happiness too.
Mother's Day is a hard one- whether it's because of a disordered mother or as a mother. It's OK to feel sad. You are not over reacting. It's normal. I hope though that this next step for you is to take a step back from this family dynamic- and have some happy moments ahead.