Hi @skachhwalverma123,
I have some information about adding another company. For QuickBooks Online, you'll have to subscribe to add a new company. It'll be one company per subscription. Unlike the Desktop version, you can add at least three company files.
You'll want to visit our page to sign up for a new subscription. Here's the link: https://quickbooks.intuit.com/ph/pricing/.
Though, you can use the same user ID and password for your sister company's account.
I'm attaching this link here in case you need help in managing your QBO accounts: Account Management.
Don't hesitate to reply if you have questions or concerns with accounts. I'm here to help. Have a great day ahead.