If your sister wants to be your donor (what a great sister btw!) I would find out roughly when you would be able to cycle, given the delay associated with compulsory counselling and other tests, and I would say she'd likely have to wean a couple of months before her stim cycle. A stim cycle takes over her hormones artificially and makes her ovulate anyway so I don't think a history of regular cycles is necessary, but weaning a couple of months beforehand will give her hormones a chance to level out. As far as I am aware, it is not advisable to BF while on FSH, but maybe this is another thing to discuss with your FS.