I have a good friend who is the youngest with three older brothers; she did indeed become quite a tomboy, but that by no means made her a "monster," as you suggest... she is actually quite a wonderful person. So I'm not sure how that would be a disaster.
That being said, also, this is TS4, and I'm not sure there's really enough depth to our sims' character development that it will make a lick of difference whether any given sim has no siblings, more siblings than the Duggars, or anywhere in-between, no matter what the gender configuration is.
But if it did, that's just more drama, and it sounds like you're already leaning into the drama for this family, so it would be a good fit.