I was worried that my MIL would get into a tizz about me having baby stuff in my house and starting to work on the nursery. However, she's been biting her tongue and not saying a word, which I'm grateful for.
I told her I was having a shower (a big no-no) and I said that she and her mother would be invited. I expected her to say no, but she said that she would love to. And her mother would love to as well. As DH said, it's all about following tradition until it comes to their grandchildren. Then all bets are off.
I think you should do what you're comfortable with. If you want to start setting up your nursery, I think that's perfectly acceptable. Those superstitions are from the days when it wasn't uncommon for a woman or her baby to survive childbirth. Things are a lot different now. Things happen, but IMO, I think it's better to prepare yourself and your house to bring home a healthy baby.