BBB -- give it some time, things WILL work out -- your mom has gotten you this far right? May not be perfect (nothing is) but things will keep getting better.
A lot of people have problems with their divorced parents getting married again -- it's completely natural. It also closes off that little hope that dad will do the right thing and make ammends with your mom. You'll get past it, as will the rest of your family.
If it's a small consolation, 4 months of courting and going into a 2nd marriage, the numbers are very much against long-term longevity of his marriage.
Hang in there. You're strong, your mom is perhaps strongest of all; be there for each other and encourage open conversation. You'll find that when the worst of your fears are stated out loud, they lose their power. Try it.