How about a little of all of the above? Mitsuru and her grandfather aren't close, but she knows all the current members on the council and she would have known of Lenna's father. And Hanae probably made Mitsuru go to the benefit on behalf of the family with her now that she's nearly a grownup. So, I'm okay with that.

How long ago was the function and how old is Lenna? These will be contributing factors to Mitsuru's level of involvment.