To be sanctified means to be 'set apart' 'consecrated' or 'holy unto the Lord.' And a transgression is sin. So, there's some confusion involved here in the term used by your sister. Perhaps what she was getting at better is not to justify sin in order to act out for 'good purposes' in it, by what you believe was the right thing to do - not consulting the Lord first in the matter (Prov. 3:5-6). Not to take advantage of God's grace basically - through Jesus' sacrifice and use God's grace as an excuse to sin (Romans 6:1-2; Heb. 10:29; Jude 1:3-4). Your works must match up with your faith (James 2:14-26). In other words, you should be led by the Holy Spirit (God's voice, which is faith) when performing any work, so that that work may be justified by Him, by your faith in Him. This is what brings glory to His name.