And Ramadhan Mubarak. Your sister's situation is a sad one no doubt BUT don't give up hope for Allah is able to do all things and everyday great advancements are made in the field of science. Perhaps one day Insha Allah your sis may very well be able to have kids and a normal family life too. So just seek out the best doctors and counselling in this field.

And always remember one's ability or inability to have kids is not a sign of him/her been blessed or wretched. For Allah gives wealth and children as a source of trials and NOT to honor them as people mistakenly believe. Hence he who has them are as trialed as the one who doesn't.
Just make lot's of dua for your sis specially in this month so full of Allah's blessings, and do all it takes.
She is still your sister come what may and will need you to be strong and supportive of her at all times. So be her source of strength always.

Barakhallahu feeki and please PM me whenever you are free. Take care!