I'm pretty sure it's intentional that Touma's sister isn't giving flawless advice here. It's a nuanced depiction of this kind of interaction, and feels very real. She's well-intentioned, of course, but she doesn't have the same perspective that Masumi has. Her expression when Masumi asks her if she could say the same kind things to someone close to her, like Touma or even her child, makes me think she's still distanced from the problem. That's a common thing, how some people will say in the abstract that they support gay people, but then have trouble accepting it within their actual family. Nonetheless, it's good for Masumi to have even some semblance of support from an adult here.