OP - you might find that your pregnancy (assuming you are older, more settled) is more celebrated than hers. You are about to be a family, your DH is going to be a family man, you are, I assume, having a baby you can afford and are able between you to raise, this is a thing to be celebrated.
You might find if she's got no job, and is known to behave recklessly, then the rest of DH's family might look at this pregnancy as a problem and a burden for them all, that as much as they publicly coo over scan pictures and sympathise about her morning sickness and sore boobs etc, they might be thinking "shit, how much help is she going to need? How much childcare will we be expected to do? She's going to be a single mum of 2 before she's old enough to buy a round, she's not going to be able to do this, we'll all have to rally round and do it for her." (even if she does turn out to be a super mother when the time comes)
You on the other hand, aren't having a 'problem' the family might have to solve, there's no nagging doubt in the back of their minds that this will be something that have to 'fix' they can sit back and enjoy yours and DH's joy. They can take the grandparent/great aunt/uncle roles without feeling they have to actually do any parenting or fund it.
If anything, you might find when you announce, she feels you are stealing her thunder.
Plus, how often do you see your DH's extended family? I only see DH's aunts and uncles at weddings, funnerals and christenings. This will be your PIL's grandchild, grandchild beats great niece/nephew any day of the week!