Why would my ex's oldest sister add me in snapchat?

To sum up, my ex left me because he said he was unhappy however in the end I found out he left me for someone else. We were together almost 3 years. He left 2 weeks after his bday. and less than 2 months for our anniversary. Apparently he got advice from her sister to break up with me. I thought the sister doesn't like me. She even knows I even block her in facebook during my relationship with him. So why would she request me? She's part of reason for the fights in my relationship and part of reason he left me. I pretty sure my ex knows I'm hurt over the breakup. I even did all my best to talk to my ex to reconsider relationship after the surprising and hurtful breakup. I truly did love him and I think he knows that but I am trying to move on. It's been a month and slowly but surely I'm moving little by little. I know he has new girlfriend so what is this request all of sudden from her sister? I mean I don't think randomly she would add by accident. Is she trying to play a game with me? I definitely won't play her game but I like to know your view. FYI Her sister is 30 years old and they both live together. My ex is 27 years old.
Why would my ex's oldest sister add me in snapchat?
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