Thanks Misao, mmm, so basically it looks like the whole imoto thing is just a plot device to add drama. We'll never see the real sister but the last episode, sort of. I would prefer the other way. This doesn't look promising to tell the truth. But I'll give it a try.
For what I saw in the NakaImo thread, I could tell that Konoe (the black-haired girl) isn't his true sister. None of the five main heroines who were shown up are imoutos to the male lead, but there's a mysterious, face-hidden girl who certainly happens to be MC's illegitimate yet true sister.
Well, yes, it could become her guilty pleasure, even if I don't think that she had problems with the genre, magical girls, but more with how hyped the show was and is. So it would be "simply" a pleasure .
I can't really speak about Symphogear because that's really not in my genre range, singing magical girls if I remember correctly. I tried the first episode.
As what I saw in your post, if ranran will watch Madoka this year, then she must lower her expectations before watching in order to enjoy. It might be her future guilty pleasure. If she will finishes watching Madoka, then she might enjoy Symphogear because of some of their similarities (dark magical girl concept, one of the main heroines suddenly dies...).