Not an exact translation of the source term, but the only English expression I have found applied to this precise situation. I think you could probably use "teaching brother", which is a fairly standard modern term, as an alternative. I have not found any evidence that this figure was called anything to do with "nurse" or "nursing".

Once they became novices they were under the care of a novice-master, but the pueri oblati were not yet novices.

"The seven-year-old "child oblates" (pueri oblati) had to submit to strict discipline at a very early age. They were under the permanent control of a teaching monk. For example, they had to sit apart from each other and were not allowed to talk or communicate with one another, let alone touch each other. The teaching monk was always present, even when the boys were washing or going to the toilet. This training and discipline lasted until the age of 15, and then came the noviciate" (p. 34)
There is a pdf of this but it refused to open for me.
http://webcache.googleusercontent.com/search?q=cache:GE6Ut_I...

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Note added at 1 hr (2013-01-03 17:31:18 GMT)
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In that case, if this "hermano nodriza" is the same as the "hermano enfermero" you mentioned to María, he could be called a "nursing monk" or "nursing brother". This figure certainly exists today:

"1968, Brother Sean was ordained a Catholic Nursing Monk where he gave 8 years of his life in service to the sick and dying."
http://www.linkedin.com/pub/barefootfranciscan-bradley-mulli...