There are two issues here.

BA Miles - used to purchase redemption tickets
You cannot use your BAEC number to collect BA Miles for your sister's flight. What you need to do is to open a BAEC account in her name (the same as her ticket) but use your address as the address for her BAEC account. You can then form a "household account" betweeen your BAEC account and her BAEC account, where the requirement is that they must have the same address. You can use all the BA miles in the combined household account, to purchase redemption tickets for yourself. You sister could also use all the BA miles in the combined household account, to purchase redemption tickets for herself if she wished knew and knows how to operate the account.
BA Miles do not reset. BA Miles will expire after 36 months of no activity on the BAEC account. One Avios transaction will extend the life for all you BA Miles for the next 36 months.

BA Tier Points -used to gain status with BA for benefits like lounge access
This is strictly for your own ticketed flights only.
BA Tier Points reset after 12 months.

BA uses the following control point. Your ticketed flight must have the same name as your BAEC account in order for BA Miles and BA Tier Points to be credited. Your Boarding Pass will have the name as ticketed and your boarding pass is checked against your ID at the gate for international flights when you board the plane.