I have two daughters and have zero plans to increase our family size. I've been asked this question and I usually say something like, "Oh, no. I've got two beautiful girls and I'm going to quit while I'm ahead." My MIL got on my case about that too. And all I can think is "WHY?!?" It wasn't MIL's family name, but FIL's. It probably wasn't your grandmother's family name either (unless she and grandfather were cousins.)
I'm the oldest of 3 girls and my parents were always being asked if they were going to go for a boy. And worst of all, it was plain that my grandmother never really forgave my mother for not giving her a grandson to pass on the family name. I don't want my girls to ever feel like that.
Marginally relevant: My friend Amy Smith married Adam Jones and did not change her name. Their first child, a boy, is Bucky Jones (dad's last name). Their second child, gender unknown, is currently on the way and the child will have the last name Smith (mom's last name). So she is a woman intent on passing on her surname (actually her father's surname) to her (second) child. Her MIL said, "Oh, are you hoping for a boy, so he can pass your last name on?"
She's a woman... passing on her last name... to her child. So if she has a daughter, the daughter could potentially do the same thing she's doing, and pass the name on. Granted, it's kind of an unusual situation, but let's think through what you're saying a little... Not to mention the whole, "Are you hoping for X?" question, which has no good answer except a sidestep.