hi, I went through this a few weeks ago. one thing that helped me was having a few plans in case they weren't supportive, and since they were, it showed that I had thought about my decision alot already.
for example, know the names of some maternity homes, get the facts on wic, housing aid, section 8, medicaid, and, if your still in school, how you plan on continuing. this way it shows that your responsible and willing to to whatever it takes to make it work.
Also, it might be good to approach it as \" I respect your opinions, but have decided against abortion and/or adoption and ask that you respect that\" if keeping the baby and raising it is the choice you have made.
as far as their behavior, a grand child might just be what they need to kick their habits. But, maybe not. Maybe you can come to a comprimise of some kind. Although it sucks, this was your decision, and although what they are doing isn't right, you can't make them change their ways.
tell her, but have a back up plan and a place to go just in case things don't work out for the best. Sometimes the best future grandparents need a little time to think and cool off without their pregnant child in their face. Your boyfreind sounds great, and although young relationships don't always work out, I have seen them happen if your willing to fight for it. So, if he's a positive and supportive source in your life RIGHT NOW, then lean on the shoulder he is offering. Some of the best fathers I know, including my own, were young ones who really steped up for their children. I hope it all works out for the best.