Praise be to Allaah.
It is permissible for this young man to propose to the girl
mentioned, because there is no impediment to doing so. The fact that his
brother was breastfed with the girl’s maternal uncle does not have any
ramifications, not even with her sisters if there are any. The prohibition
affects the one who was breastfed (which in this case is his brother) and
anyone else who was breastfed by the same woman.
The young man mentioned here is not one of these people.
With regard to the older brother, if he was breastfed by the
grandmother of the girl, then he is a maternal uncle to her through
breastfeeding.
And Allah knows best.