The general rule of thumb is to have a close mehram from the father’s side breastfeed if the child is a girl and a close female from the mother’s side if the child is a male. Generally the milk relatives are the same as the biological. Your meharams are your parents, grandparents and their parents …, siblings, parent’s siblings, siblings’ children, children and children’s children …
The siblings will become mehram of the child and the parents this way. According to most schools of thought the breastfeeding has to be done within the first two years of the child’s life at least once to a minimum of five times.