Only a US citizen 'spouse' can file when under 18. Otherwise he/she must be 21 to file for mother/father/brother/sister.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
This answer is for general information only and is not intended as legal advice regarding your specific situation. In order to assess your legal matter properly and obtain legal advice, you must contact an attorney directly. Provision of this general information does not create an attorney-client relationship.
Unless you have been maintaining a valid immigration status, you cannot adjust status while remaining in the US. For immigration purposes, your sister is not your immediate relative and you must be in certain valid immigration statuses to adjust status while remaining in the US.
If you "came in with a tourist visa 15 yrs. ago", you likely are not in a valid immigration status now and likely have overstayed your authorized stay by at least a year. If you leave the US, you will be barred from re-entering the US for at least 10 years.
If you marry a US citizen for purposes other than to gain immigration benefits, your overstay would not be held against your petition to adjust status.
You should review your specific facts with your attorney to see what legal options you may have.