Your sister may also be going through post partum stuff. Her body is trying to get back on track of hormones and so on to where it was before being pregnant. When I went through it I snapped at people, in and out of depression, didn't know from one minute to the next what I wanted to do so I either just agreed with everyone or shoot their ideas down flat. Give your sister some time its not you and post partum stuff has no time limit some people are back on track in a month others it takes sometimes years. Since there is no telling how long your sister is going to react this way the best thing you can do is take care of yourself and try not to take anything personally. Hard to do I know. good luck.