I haven't read the replies, but yeah, at 3.5 I think my twins are harder than one 3.5-year-old, or, say, one 3.5-year-old + a baby, or + a 5-year-old. Having two children at the exact same developmental stage is difficult, because you can't even count on one to be a little more mature than the other, but that's not really all it is. When I interact with them, I don't feel like I am just interacting with two people. I'm interacting with Lilly, and I'm interacting with Kate, but then there's this third entity that arises from the combination of their two personalities and their perpetual intertwining. It's that third entity that makes it hard. All siblings play together and fight together, but the intensity with twins (at least with mine) is just *more.* Maybe it's just my kids, but I sort of doubt they're that unique.
On the other hand, if I was not a twin parent, I would find it annoying if someone constantly went out of her way to remind me that twins are harder. That's just irritating. So, I think your SIL is right but I also think she should shut her mouth about it.