do a swatch first; there's no cotton in the Sidar, but there is in the twist; that might make a difference when it knits up. (i'm thinking there would be a "support" difference, thought I can't imagine a knitted bra would give any support anyway to milk-filled breasts, but I don't have kids so what do i know?)
Also, be sure to calculate how much yardage you'll need, so you buy the right amount of yarn.
Sidar = 347 yards per ball
Twist = 99 yards per ball
So, 3.5 balls of twist = 1 ball of Sidar yardage wise.... If the pattern calls for 4 balls of Sidar, you'd really need like 14 balls of the Twist.
14 balls of Twist * 5.49each = $76.86
4 balls of Sidar * 7.95each = $31.80
... meaning you could make 2 and a half bras out of the Sidar for the same amout of money as one bra of the twist. (or 2 bras and a matching bandana)
AND, with the Sidar, 347 yards = 100 grams, but 99 yards = 50 grams in the Twist, OR 198 yards (2 balls) of the twist weights as much as 1 ball of the Sidar, but still has less yardage. This is a big weight difference, and might not matter with a bra, but in a sweater, for instance, one would for SURE be warmer than the other.... and in this case might add to the support issue as previously mentioned.
Anyway, thanks for the excuse to micro-analyze numbers. math rules.