You cannot remove a SSD containing Windows 7 from another computer and install it in your newly-built computer.
You need to install Windows 7 in the SSD in the computer it's going to be used in.
But there may be a problem doing that in your newly-built computer.
It's my guess its hardware (motherboard, processor, etc.) is too new to support Windows 7.
Hopefully, you can re-install that SSD back in your sister's computer and Windows 7 can still load and work.
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