Let her know you feel a little left out. Plan a day where you too can do something together, get a babysitter, and have a mom and daughter day. Get your nails done, see a movie, get a bite to eat, etc. Also, if you help her out with your siblings just a little bit, it'll take some stress off. Like maybe 2 or 3 days a week, take your sister on a 30 minute jog. And maybe help your mom cook dinner one day, etc.