。
She had been planning to visit her hospitalized mother.
Literal
She [topic-は] [in-hospital-入院中's] mother's visiting-the-sick [to-に] go [intention-つもり] was [explanatory-んです].
見舞いに行く ('go to visit the sick') is a stock phrase, with 見舞い as the noun for the visit and に as purposive. 入院中 uses ~中 ('in the middle of') as a noun-modifier — 'mother who is hospitalized.' The closing ~んです (explanatory) + ~つもりだった (was intending) gives a tone of providing background or excuse.