Treat them equally. How could you not?
I'm assuming if you could afford to pay for all of one DD's wedding, you would have made provision for the second DD.
You said that your elder DD invited quite a few of your friends. And your DD wants a smaller wedding and so probably won't ... is that why you think you don't need to contribute? The younger will have a smaller wedding, so it will cost much less.
She's probably having a smaller wedding as she does have a mortgage and your grandson to care for. A smaller wedding would be a much more intimate affair and probably just as much fun. If you want a party with your friends, you could host that in the run up.
I paid for my own wedding, and my MIL invited quite a lot of her friends/relatives My DM found out and invited a substantial amount of hers at the last minute. I barely knew them but wasn't good at standing up for myself in those days. We had a tiny flat and were broke for a long time afterwards, with both of us taking on evening jobs as well to make ends meet. I have to say given their total lack of contribution, I resented the selfishness of both in doing that to us.
If you don't know whether your DD wants a contribution or not - ask her. But I'm guessing with paying for a new home and taking care of her DS, probably paying for childcare, that she may well need it. Also, it might delay any future plans to have another child if she gets no contribution at all. It's good that she's getting married tho to safeguard her own security and her child's future.