Some states allow for grandparents rights. Not sure which state you are in and if there is an option there or not.
They are missing their daughter and their grief is causing them to want to hold onto the one thing she left behind... her son. They probably don't know what is driving them, or probably don't want to admit if it it crossed their minds. It's not rational. When a person loses a child, even if that child is an adult, it doesn't matter. The grief is horrendous!
If you are doing everything right, it's not likely they will win custody. One thing you can offer them, after they lose, is the proverbial olive branch. Tell them you are happy to have them in your sons life and that you are happy for them to visit. But any further attempts at custody and all of that will make you feel less inclined towards them.
If it were me, I'd offer them a visitation schedule (through the lawyer only!). See if they'll take it instead of a custody suit. A visitation schedule between two parties is not legally binding unless you have a judge sign it, which is something I am not suggesting! Just a nice schedule between two people that a lawyer penned up to leave you with all the rights. To leave you the option of denying the visitation if needed. Like if the child is sick or something. But a good reason and never a vindictive one.
It's a shame all of this happened. Just try to accept how much pain her parents are in. I doubt highly they hate you. But it must be hard to see their grandchild with his dad and new girlfriend. They probably feel like the new girlfriend is trying to take the place as mom, take the place of their daughter. Can you consider that? can you imagine that? Imagine if you died and your ex had a new boyfriend already. Wouldn't you feel like he was going to try to take your place? Try, just try, to understand it from their side, even when they don't get it themselves.
Make certain you have your parents stand up in court and explain they will be there for you and baby. Working two jobs and going to school makes it seem like you won't have enough time to raise a child, so make certain you have a great support system in place! do it right away.