My twins are 13 month n still not walking is it normal (born premature at 33 week)?

Normal. The upper end of normal for a term born baby is around 15-16 mo for independent walking.You add 7 wks to that for your 33 weeker. By 2 most have caught up with term born kids.
Yes. Yes. It should be fine. I assume they were born premature but with no problems. Some start taking few steps as early as 9-11 months. Others might have a little delay beyond that. Bottomline, if everything else is ok and you're regular with your pediatrician visits and vaccines you should be fine. It is only a matter of time.