Board Thread:General Discussion/@comment-31344849-20170219160624/@comment-26887879-20170219162011

I mean, dialogue with echo flowers could be done by anyone. There are much more characters in the underground than the ones we encounter, there could be other characters that have a sister. We have no evidence that it's MK talking or any character we know in that case...

And if MK's sister was ereased from timeline with Gaster, then wouldn't MK forget that he ever had a sister? Because he does mention her in game.