Yes, that’s fine. You can use someone else’s SAIL address, including that of your sister company, as long as it is situated in the same country as your registered office and you have permission to use it.
If you decide to go ahead with the use of a SAIL address, you are required to tell Companies House immediately. You can do this online by submitting form AD02 via WebFiling, or you can login to your client account on your company formation agent’s website. Both options are free.
In addition to notifying Companies House of the use of a SAIL address, you will also have to tell them what records you are moving to the SAIL address. To do so, you can file form AD03 via WebFiling or provide this information through your client account.