I wouldn't take it as an insult. In fact you might be genuinely supportive of it. Here are some things to consider.
You said he's a much better photographer than you. Once in awhile, wouldn't it be nice to have some really good photographs of your girls?
Your wife may be trying to save you a bit of effort. She wants Studio Photos. It isn't that she doesn't want your photos. If you have a DSLR and she's supportive of that, I'll bet she likes - and perhaps even loves - your photos! Can you do not just studio photos, but the consumate Studio Photos? It's like going out for a steak dinner. Sure, you can grill some steaks at home, but is it really an insult to go to a fine restaurant to let them prepare an even better steak for you?
Here's an idea for the future. Do you print your photos? If not, strive to print some and frame them. Make photo books. I just finished one of a vacation we took in November. While I'm not the best photographer in the world, we have family memories now in a hard cover book that will last for years. I have photos of my wife and daughter, as well as friends from here we met while away.
Your wife's not putting you down, nor is she putting down your photography. She wants Studio Photos. Say Yes with grace.