this may be senstive topic to those who have strong opinions. im flexible, im only mostly concern about keeping up with grammar level. sign language is actually communication for the deaf people, not speaking/hearing (maybe for those vareity of hard of hearings). this is how we can have conversation easily and very understandable. sign language is pretty important to us. so i think using SEE for the kids would help to develop their skill in writing, using every words. this way, they'll grow up having no problem with english while socilizing with others at older age they'll learn some ASL and use mixed SEE and ASL (i dont know what is it called right now)
this kind of language (mixed ASL and SEE) is what im using now. hope this helps for the best for your sister.