I am leaning towards the opinion offered by others, that Johnto probably was a lover rather than a brother. Perhaps he may even be the man MJK took to sleep at Mrs Carthy's one night if this 'brother' really did visit her in Whitechapel?
Belatedly, yes that was my feeling too, hence my query...in particular the London connection (though he may've just spent some duty time or leave there). It's still all a bit "thin" at this time distance though isn't it? (Of course just about everything is though really!). Must drive you mad sometimes!
All the best