FAQ

How many Postpaid family lines (supplementary) can I sign up for?
Each principal can sign up to six (6) postpaid supplementary lines which can be a combination of the below postpaid supplementary plans and subject to the principal plan eligibility:
Postpaid 40 can sign-up Family 40 supplementary lines only.
And right now Digi give promotion half-price for each supplemetary lines.

So your case will be RM60 total before add-ons and taxes.
Be advised if you sign up for internet sharing facility (add-on RM10), that Digi supplementary line will withdraw data from principal line's shared quota 1st before consuming its own dedicated quota.
You might find yourself suddenly out of quota, sucked dry by supplementaries! You can overcome this default "share all until dry" behaviour by assigning tiny limit through MyDigi app. Can't disable altogether.

???

Principal and supplementaries are single account. Of course they will be single-billed together with the principal. All telco give the same treatment.

If you want separate bill, then sign up as new principal account either as her's or your's, not as your supplementary. Of course, you will need to choose non-supplementary plan.

???

.... of course you cannot add her immediately since both numbers are under different IC. She have to transfer her number into your ownership, then only can you merge into pricipal-supplementary relationship.

based on your screenshot, that screen is for you to select a new number as NEW supplementary, NOT make existing number as supplementary.

and as it is, your sister number is not even yours to handle.

you should go transfer ownership first, then separately deal with making it a supplementary.

aiyaaaa.... of course lah. would you like me to claim ownership of your number and make it my supplementary without you even knowing what happened?

you should go transfer ownership first, then separately deal with making it a supplementary. i don't think digi website have interface to execute that, you'll have to walk-in since both sides have to present ID for transfer verification.