Yes, it certainly is important to pray for others. If Jesus thought it was important for Him to pray for others, shouldn't we do the same?
Does this mean God will always answer our prayers exactly the way we think He should, or when we think He should? No, not necessarily. We only see part of the picture, but God sees it all, and even when we don't understand His ways, He can be trusted to do what is right. The Apostle Paul told one group of Christians, "Since the day we heard about you, we have not stopped praying for you" (Colossians 1:9).
Continue praying for your sister—and pray for her husband also, that he will be convicted of his irresponsible behavior and begin to live the way he should. Pray also that she will have wisdom to deal with her situation, and that she will turn to God for the strength she needs. The Bible says, "The Lord is a refuge for the oppressed, a stronghold in times of trouble" (Psalm 9:9).
In addition, ask God to help you be an encouragement to your sister. Yes, she needs practical help—but she also needs to know others care about her. Most of all, she needs to know that God cares for her and wants to help her. May you both discover the joy and peace that comes from trusting Christ, even in the midst of life's turmoil and uncertainty.