Wow, the sister is an asshole.
"I have a lover in university, I leave them, and my sister, to make merry in a foreign country, meet some stud there and decide to marry him"
Talk about being selfish and insensitive.
Wow, are we reading the same manga?
The sister met and dated the best friend back in college. She is now a working adult. There's plenty of time there for a break up. Also, it's clear that the sister and best friend broke up on good terms, seeing how they're still communicating with each other.
And yes, I know this is a yuri and any female character marrying a male character is pretty meh, but love happens.
Anw, seeing how the sister took care of MC in place of their parents, still reached out and care for MC after she left home, plus having a good relationship with her ex-lover, I'd say that the sister seems like a really good person, not selfish or insensitive at all.