SLEEVES (Both alike)
Using 3mm (USA 3) needles cast on 33 [33: 35: 35: 39] sts.
Knit 6 rows, ending with RS facing for next row.
Change to 33
⁄4mm (USA 5) needles.
Sizes 6-12mths and 12-18mths only
Next row (RS): K1, (Inc in next st, K1) 16 times. 49 [49: -: -: -] sts.
Sizes 18-24mths, 2-3yrs and 3-4yrs only
Next row (RS): K1, (inc in next st, K2) – [-: 1: 1: 3] times,
(inc in next st, K1) – [-: 13: 13: 9] times, inc in next st, (K2, inc
in next st) – [-: 1: 1: 3] times, K1. – [-: 51: 51: 55] sts. All sizes
Beg with a P row, work in st st shaping sides by dec 1 st at each end
of 4th row, then 3 foll 4th [4th: 4th: 6th: 6th] rows. 41 [41: 43: 43: 47] sts.
Work 5 [5: 7: 7: 7] rows more, ending with RS facing for next row.
Inc 1 st at each end of next and 3 [2: 1: 2: 2] foll 4th [4th: 4th:
4th: 6th] rows, then every foll 6th [6th: 6th: 6th: 8th] row to 53 [55: 57:
59: 61] sts.
Cont straight until sleeve meas 19 [22: 24: 27: 29]cm, ending with
RS facing for next row.
Shape top
Cast off 4 [4: 4: 5: 6] sts at beg of next 2 rows. 45 [45: 47: 49: 49] sts.
Dec 1 st at each end of next 5 [5: 7: 7: 7] rows, then on 2 [2: 3:
4: 6] foll alt rows, then on every row to 17 sts.
Cast off 3 sts at beg of next 2 rows. Cast off rem 11 sts.
I am knitting size 2-3 years brackets are confusing me is itvx new row with each increase ?
SLEEVES (Both alike)
2-3yrs
Next row (RS): K1, (inc in next st, K2) 1 time,
(inc in next st, K1) 13 times, inc in next st, (K2, inc
in next st) 1 time, K1. 51sts. All the increases are across a single row.
All sizes
Beg with a P row, work in st st shaping sides by dec 1 st at each end
of 4th row, then 3 foll 6th rows. 43 sts.
Work 7 rows more, ending with RS facing for next row.
Inc 1 st at each end of next and 2 foll 4th rows, then every foll 6th row to 59 sts.
Cont straight until sleeve meas 27cm, ending with
RS facing for next row.
Shape top
Cast off 5 sts at beg of next 2 rows. 49 sts.
Dec 1 st at each end of next 7 rows, then on 4 foll alt rows, then on every row to 17 sts.
Cast off 3 sts at beg of next 2 rows. Cast off rem 11 sts
These parentheses and brackets can be confusing. It’s easier if you circle or highlight the numbers that apply to your size.
In the initial instructions you are increasing across a single row. Then the decreases are at each end of the row, so the beginning and end of row.
Some of the decreases are at each end of the 4th then 6th rows.
The increases that follow are every 4th row and then every 6th row.
When you get to the cap The decreases to shape the cap are again at each end of a row.
Marking off your size will make this clearer and make the knitting easier.
What is the name of your pattern?
The pattern is peony in Rowan. Just didn’t seem right to do all increases in a single row I thought it would have been over 13 rows 
It looks like the sleeves blouses out after the first 6 rows of the cuff. This is due to the change in needle size and the rapid increase in stitch number. (The stitch number count here appears to be off. 35 + 15 = 50sts.)
What I done to get the correct number of stitches was
K1 Inc 1, k2 Inc 1, k1 Inc 1 till last 2 stitches then knitted last 2 hopefully it will work out correctly. Thanks for your help never known such a rapid increase before thought I was reading the pattern incorrectly.
As long as you get to 51sts you should be fine.