I don't see why it's insulting. She's family. It's not that weird for a couple to live with other family members, especially in Japan. The epilogue makes it clear that they've got a pretty good relationship at that point, and it doesn't look like she's going to get in the way.
I don't mean insulting as in 'the writing quality is insulting', as I can totally understand why they would allow the sister to stay with them. I merely meant that I personally really dislike the sister and so it makes me mad that they'd let her stay. Apologies for the confusion.
Little Hana doesn't deserve live without her mother, so all right.
The fact that they didn't immediately kick the sister out after moving in together is insulting.