please help me 2 separate accounts merge under family account?

Microsoft accounts and Microsoft 365 subscriptions cannot be merged or combined into a single account.

What can be done instead:

1. Keep Microsoft 365 Family as the main subscription
   • Use the Microsoft 365 Family subscription as the primary one.
   • Share it with the second account (for the sister) so that both accounts use the same Family subscription. Guidance for sharing is in the article about how to share your subscription.
2. Cancel or let Microsoft 365 Personal expire
   • Because subscriptions and purchases cannot be transferred between accounts, the Microsoft 365 Personal subscription cannot be moved under the Family subscription.
   • To avoid future double billing, cancel the Microsoft 365 Personal subscription or turn off recurring billing so it does not renew again. Use the “Cancel subscription” or “Turn off recurring billing” options described here:
     • Cancel subscription
     • Turn off recurring billing
3. About refunds
   • Refunds for subscriptions are handled according to the refund policy of the store where the subscription was purchased.
   • For Microsoft Store purchases, see the guidance in Help with refunds.
   • If the Microsoft 365 Personal subscription was purchased through another provider (for example, Apple or another reseller), request the refund directly from that provider using their refund process.
4. No account merging
   • It is not possible to link, merge, or combine Microsoft personal accounts, and items such as purchases, subscriptions, or balances cannot be transferred from one account to another. The two accounts must remain separate, but they can be used side by side.

References:

• Combining Microsoft accounts
• How to resolve being billed on 2 separate charge accounts for the same thing - Microsoft Q&A
• Family 365 account subscription error - Microsoft Q&A