I was super picky with my kids shots and you can get all of the pictures you want with either the 50mm f/1.4 or something the like. I would suggest you get the 40mm pancake lens for your baby shots. That way, you also have the ability to use it in video and have autofocus during your videos. You little one isn't going to move too much at all for the first couple months, they will just sit there and look cute. Therefore, you don't need the top of the line lens for chasing them around. You can look into that sometime down the road. The 40mm gives you great image quality and is very portable on your camera. On top of that, it has a very good minimum focus distance, so you can get those detail shots that you want. If you have a little room, you can get good family shots of 3 or 4 people with the 40mm. Here is the Amazon link. Here's Bryan's review.

You can use your 15-55mm for the big family shots, just make sure you have plenty of window light.