I've known a few people in your situation. I can say in all of those cases, after the mom and dad get over their initial feelings- you know, embarrassed about their child having a child out of wedlock- all of the moms and dads were happy about having a grandchild, especially if it's the first grandkid. You are going to have to decide when the time is right to tell them. Since your sister has not told you about her wedding (seriously, WTF?!) don't bring it up before then.
But I have to say, Congatulations!!!