I think people can hold in gas until they go into a different room.
And perhaps *sometimes* a person can control how the gas comes out.
And when it comes to burping, I do know people who can deliberately make themselves burp.
But I've personally never been able to deliberately make myself fart. Not from scratch, anyway.
If I'm in my home, I don't necessarily want to hold it in all the time and walk into the bathroom to let it out.
The gas is there, and perhaps I can influence how it comes out (i.e., speed things up, which may also, as a side effect, make it louder). But if I ever do that (and I have), it's not because I'm trying to be gross but because that gas is uncomfortable in its current position, and getting it all out at once is probably going to be more comfortable in the long run.
I'd bring up first any concern about her health--excess gas is not a sign of health. And with a family member, I think you can absolutely say, "Would you please make an effort to step into the bathroom before you pass gas? It smells amazingly awful, and it grosses me out to hear it."