Why does this topic of supposed topless women breastfeeding babies keep coming up? I have never seen a woman feed her child like that in front of other people. It just Does.Not.Happen.
Yes. OP, can you clarify what "pretty much topless" means? Is she wearing no top at all, openly? I could see your maybe having an issue with that. (Still doesn't let you tell her what to do in her house, but if you wanted to urge your sons to leave the room, leave the house, or quietly let her know that you're not comfortable with them being in the room when she's disrobed, that would make sense.) Or does it mean, "wow, you can see her breast when she does that"? In which case, well, that's part of breastfeeding. She's comfortable with it; help your sons be respectful of it.
There are a lot of scary things in the world that they're going to come into contact with as they grow into adulthood. Breasts are far, far from the top of that list.