No excuse. Explain that before mobiles/text facilities time meant time! Goodness me! when my parents told me I had to be 'in' that was it, otherwise I was grounded (only we didn't call it that then
). Explain to your sister that you are fed up with taking advantage of. That by changing her plans yours are altered and that impacts on your time with your husband.
Time you began putting him first. Otherwise ........... I'm telling you this because a friend was in a similar position, his wife spent hours with her family, less time in the family 'home' .......... he ended up caring for the 3 children, pets, school runs so eventually he walked, taking them all with him. She felt that her step-father required her time and that her husband would be 'happy' taking over the household .......
So - take care of you
. Once you have put your foot down with a firm hand your depression will seem easier to deal with too!