We as Latter-Day Saints - aka Mormons - do believe in the Law of Tithing as mentioned in the Bible (Malachi 3:8-10). We believe in donating voluntarily 10% of our increase. So yes we believe in tithing. However it is voluntary and no one is forced to do anything. We do it because we love the Lord. Also there are great blessings that come with it. Quoting Malachi ".... prove me now herewith, saith the Lord of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it." My family and I have received many blessings that we feel come to us in part because we live the law of tithing. 
 
I'm sure your sister will receive many blessings from the Lord as she lives the law of tithing. 
 
One of the uses of tithing is to help those you are poor or less fortunate. Here is some info about that: Link

Yes. The tithing is used for various purposes, including helping those who are less fortunate and humanitarian efforts around the world--regardless of race, gender, or creed.

Tithing is honoring god.

Yes, one of the covenants made during Baptism is to live according to the law of Tithing. When you are interviewed for Baptism you are asked if you are willing to live by the law of tithing.  
 
The Church will not excommunicate a member for not tithing nor will they force anyone to pay there tithes, but not being a full tithe payer has impacts on ones standing in the Church (such as calling and receiving a temple recondite). 
 
I would like to point out thought that the Law of Tithing is a lower law, based upon the Higher Law of Consecration, where one gives 100% of there income and property to the lord. The Ancient Church lived under this law as well as the early restored Church, unfortunately the membership failed to be able to live this Celestial Law of Consecration and lost many blessings, most importantly the loss of New Jerusalem or Zion, and the lower law of tithing was reinstated.