It is obvious that your sister may resent your conversations about a topic that may be quite painful to her. In fact each time you bring it up you may be opening and rubbing salt in an old would for her.
Try dropping the subject with your sister and find someone else to discuss pregnancy and your desires for a baby with at least for a while.
Once you become pregnant expect a mixed reaction from her at first. Of course she will come around and be supportive and love and cherish the baby. Sisters always do.