One of the most effective ways to help your niece is to read easy materials together. As you both read aloud, run your finger under each word as it is spoken. At first, you may have to repeat the first lines or paragraphs several times until your niece is reading in a normal fluid fashion. Read together for 10 minutes several times during the weekend. As your niece's reading improves, begin to read more difficult material. Hopefully, she will soon be able to read some of her school books.
Begin to help your niece who can't multiply by helping her understand the concept of multiplication. Place two beams or some other object on each of threes plates to illustrate the problem 2x3. Repeat this activity with different combinations. Next, use flash cards beginning with the 2's. If your niece doesn't know an answer to a problem like 2x4, she should draw 2 horizontal lines and then draw 4 vertical lines through the horizontal lines. By counting the number of intersections, she will get the answer 8. This approach lets her find the answer through her own activity.
The Department of Education offers help in teaching children how to read and do math problems through its publications at www.ed.gov.