Your sponsor is spot on. You have to put your recovery first. If you are having trouble saying no, let your sponsor guide you. When I was a kid and didn't want to do something I was being urged to do, I made my parents my heavy. Say no but if she asks for a reason tell her your sponsor says it's not a good idea.
If Betty has a pattern of doing this, don't become part of her pattern. It won't be doing her or you any favors. What if she keeps using? That stress alone is bad. I have fifteen months and am dealing with a husband in relapse and believe me that it is hard for me to just not say forget it, I'm going to drink. If it were easier to kick him out, I would do so. Betty isn't in your house. Don't let her in. It's a consequence. Think of it that way. And if she keeps having people rescue her from the consequences of her own choices she will keep doing the same thing. Your own sobriety is at stake. Say no.