How do I stop my SIL from meddling?

So my sister-in-law has this habit of showing up unannounced, and honestly, it’s getting out of hand. Last week she walked in while we were arguing about money — not full on fight, but just trying to figure stuff out — and she starts giving my husband advice like she’s some kind of marriage expert. Like, we didn’t ask for it. And he just nods along, even changed his mind about our budget because *she* said we were being too strict. It’s not just that. She texts him daily, asks where I am, what I’m doing, and now she’s even commented on how I discipline our toddler. Said I’m "too harsh" because I made him sit in timeout for throwing toys. My own husband told me later she had a point?? I’ve talked to him, but he says she’s just trying to help and that I’m overreacting. idk, maybe I am, but it feels like she’s part of our marriage now and I didn’t sign up for that. We’re thinking about separating, not just because of her but this is making everything worse. How do I get through to him that her being in our business isn’t normal?