Saw FIN. (So: spoilers.) I'd give it B- by its own merits, hovering somewhere around HLV or TBB. The biggest problem was I simply didn't care about the sister. I also didn't sense much connection, at all, to what John and Sherlock had been going through since TRF.
I'd like to suggest an alternate way of doing what I think this episode was aiming for: make Mary Moran. Don't kill her off, and come up with some suitable reason why she finds herself at odds with John and Sherlock. Don't make her evil -- make her conflicted but ultimately a threat to them. To protect Rosie, herself, someone else. Or give her an ideological reason or, heck, *do* make her Moriarty's heir.
And use that to make John and Sherlock come together, go through a crucible and come out the other side as the team they were at this episode's beginning by some quirk of fate. Also, rather than making Molly... this travesty, why not use the fact that she betrayed Moriarty and forced Mary into this position. Use Mycroft's complacency in using "consultants," give us a reason why he stopped, have him be the one who put Mary in John's path for his own reasons.
Goodness, but there's such potential here if they'd just fallen their story through to its natural run, actually used the canon, actually written female characters worth a damn. You could even roll Euros into it, as a sort of partner for Mary if you wanted to do the complicated family thing.
I am... not thinking about Johnlock issues here. Can't make myself go there, and really you don't have to. Because there's just so much potential here, even without going there. Really --*really*-- such a waste.