Cis girls take 5-10 years to develop, and there's no reason to expect that our breasts will act differently. There are plenty of girls who are still less than an A cup a year into puberty, which is a more realistic way of looking at it. Speed does not determine final size, and it's worth noting that the speed of development is a different genetic factor from the final size, too. (So, for example, if your sister was like my wife, who started puberty at 11 and wasn't finished growing breasts until 22, you might also have inherited the slower development pattern.) I know it's hard to be patient, but everyone's bodies are different.
I started on patches with no T blocker, as you have, and I had relatively quick development; my sister was also one of those girls who went from flat to C cup in about a single school year. The patch method of administration and the lack of T blocker *can* be effective, particularly since there's no point in using a T blocker when your T is already in the female range. Estrogen is also a T blocker to some degree; the main reason most doctors don't rely on it exclusively anymore is that most trans women require very high doses of E to keep T under control. Since you clearly do not (you're on patches and your T is in the desired range), all a separate T blocker would do is lower your levels *below* the preferred range.
