Elementary tattoos

So huh, Sherlock has a back tattoo which reads:

Sister
Mother
Father
Brother

So I'm guessing there's one family member yet to be seen?

(I'm at S05E19 so don't spoil if she's coming soon.)

Other tattoos:
"MT [?] Gangster" with a sunglasses-wearing skull
26.2

Anyone know/remember what these would be about?