。
Mother is making us tea.
Literal
Mother [subject-が] tea [obj-を] is-making-for-us (polite).
お茶を入れる is the idiom for 'to make tea' — literally 'put in tea,' from the action of putting tea leaves in water. Note the unusual verb choice: 入れる ('insert, put in'), not 作る ('make'). The ~ている on the benefactive ~てくれる gives the ongoing-action reading — 'is currently making it for us.'