We broke up almost seven months ago. But my ex-gf and my sister are still on really good terms just like before. Recently, I informed my sister about my breakup. I didn't really go into the details of the breakup like mentioning BPD because 

-&gt; She's always been exceptionally nice to my sister and nephews. 

-&gt; Importantly, where I'm from, not a lot of people have any idea about personality disorders.

I informed my sister that it just didn't work out due to irreconcilable differences and I had to break up. My sister is cool with that. Now, even though they don't call and text each other frequently, they still wish each other on special occasions like birthdays, and festivals(we have a lot of 'em here in India). My sister really likes her because she hasn't seen the other side of my ex-gf like me. Also, my nephew is really fond of her. Today, my ex-gf texted my sister to wish my nephew happy birthday. My nephew and sister replied to her with voice messages. They were happy about it. I started to wonder if it hinders her from moving on or she considers this as something else. She started her therapy recently like a couple of weeks ago. I want nothing more than for her to get better. So, I suggested my sister to not talk to her. My sister's response was "I never really text her. But it's basic courtesy to respond when someone wishes you on special occasions. I don't even know why you are mad about it. She's such a lovely and well-mannered girl. But if you insist, I'll block her." My sister would do anything for me. She's like my best friend but I felt like I'm being unreasonably paranoid. Maybe my ex doesn't have any underlying motive and just wanted to wish. I'm not sure. She never asks my sister anything about me. She doesn't try to contact me through my sister. (But she's been trying to contact me by texting and calling. I have blocked her everywhere). All of her interaction with my sister seems harmless and nice.

I wanna know is this something that I should worry about or am I just being paranoid? I feel like this sub is the right place to get proper answer for this doubt in my mind. Please enlighten me. 

PS: Sorry if I come off as a stereotypical person who hates people with BPD. Because I'm not. I really want her to get the help she needs and be happy in her life. My only concern is what if she feels like there's still a hope for us and that stops her from moving on.