I opened a joint investment account (GIC) with my sister in Canada. On the tax form T5, both of our names are written. Could someone help me how to file tax?

If the person that you are splitting your T5 with is not your spouse, then you'll enter only the portion of the T5 split amounts that are yours. In your case, if your T5 is for $1000 in interest, and you are splitting 50/50 with your sister, you'd enter $500 on your return, and she would put $500 on hers.