Not sure when you turn 18, but getting married, having a baby, preparing a sponsorship and getting together the funds to send it off (1000+) could very well take some time... It's taking some people 12+weeks to get FBI reports back and sometimes they come back inconclusive due to poor fingerprints and need to be redone... He will need to get a special medical and the two of you will need to gather and organize your relationship evidence (including your Childs bc since they will probably be born before you can get everything together../ heck all the things you need to do, you very well could be 18 before you apply even if the rule didn't exist... But it does

And the only real way your mother could sponsor him on a reasonably fast track would be as a dependent SON (not son in law) which would involve her adopting him and him being your brother... Ewwww!
Good luck and Congrats.. Lots of changes and hard work ahead sounds like you are very lucky to have the support of your family!