Would it be a sin for someone in RCIA who is not Catholic to not attend Mass on a holy day of obligation?
No, it wouldn’t. However, if you can attend it would be laudable and helpful for your journey of faith. During this time in RCIA it’s good to immerse yourself into Catholic practices because Catholics practice what they believe. You should know that even Catholics who couldn’t attend are not obligated to attend due to work schedules, caring for ill family member(s) or small children, or are sick themselves, etc. If in doubt you can always ask your parish priest about it.
No, you are not under obligation. However, RCIA instructors encourage attendance, as it will assist you in your understanding of the Mass in conjunction with your courses.
No. And neither is it necessarily a sin for a Catholic…more is involved than simply going or not going.