I was surprised that no one seemed to mention the impact of large families on the earth. What about population control and lessening our footprint?
To the OP, I would point out that Christmas is not the time to talk to anyone about being a parent, because it tends to be the season where most people are most negative about it. It is- or can be- the season of crass commercialism, of whinging, of children asking with total seriousness if Santa has an email address so they can change their Christmas list on Christmas eve, and the cost of taking five children to see a mall Santa gets prohibitive. If you look, even some of the die-hard NFL advocates have posted about stresses with gifts. It's just THAT time of year.
I'm pregnant with my fourth, and I know that I could not be the parent that I am if I hadn't had a big gap between 2 and 3. Child spacing can be as important as family sizing, and I did notice that your BIL's family are very evenly spaced, with a new babe every 3 years or so. I couldn't do that. I NEEDED to know that my boys could make their own breakfasts and wouldn't starve if they were hungry at the same time as the new baby. I also needed the maturity- I'm a better mum at 30 than I was at 21. In the end, though I have regrets about my life, choosing to have, or to keep, each of my children is REALLY not an issue.
Oh, and I'm not a full-time mummy. I'm just me. I have three kids, one still gestating, a burgeoning interest in fibre crafts, an obsessive interest in emigrating to Mars and I have a crush on Heston Blumenthal : I'm not working at the moment, though I have done in my life post-parenthood, and I do a lot of voluntary work with families and young children. I'm as much myself, if not more, than I ever was, and I get a lot of love and support to follow my dreams from my family, just as my kids do. If you're interested in the whole "losing my identity" issue surrounding motherhood, I can recommend some fantastic books on the politics and feminist ideology surrounding motherhood that I think you'd enjoy.