No, SIPs doesn't require any demat account. One can start SIP by completely minor documentations such as KYC, FATCA etc.
No, you don't need a Demat account to invest in a mutual fund, including SIPs. Demat accounts are only mandatory for executing online transactions of shares on the stock market.
If you are just looking to invest in a SIP, you can do so by directly visiting the website of the fund you are looking to invest in. Apart from that, you can also have a broker or financial advisor to help you sort out the initial process to start a SIP.
Nowadays, various platforms are available where you start your SIP on your own by completing very minor formalities such as KYC, FATCA, and so on.