But there's a huge difference between "substitute" and "replacement," which is what the MILF and Sammy Davis are actually about, but if you admit that you're admitting that people don't owe their existence to you, and if Nancy thought that was true for a second she really would lose all control. Mothers and sons have been doing this since Seth and Abel and Cain; single mothers have it one thousand times harder, widows twice that bad, and that's just the normal bits. Every family is different and it's always hard to know what is universal and what is specifically an issue with your specific family, and in this case you couldn't blame Nancy for being weirded out about not knowing which is which, because all Judah did was die. She's done way worse to them.
This is a tiny scene but in some ways it's the big one, because she's fighting a war on so many fronts: he's growing up, he's putting the drug thing between them like always as a way of asserting his eldest-boy freedom, he's resentful as hell but still young enough to need his mom, which also galls him, and that's all normal shit. Add to that Shane's concomitant freakout, the fact that Silas's already-troubling sex is with a MILF, the fact that they're all drug dealers, the fact that he's now a totally independent contractor doing the dumb shit that Nancy was doing back in Season 2. (Fast-forward to Sanjay falling in love with him and burning down the Cheese Shoppe.) And all of this mom stuff, and drug stuff, and then trying to have a relationship on top of it, and on top of all of that, trying to be a woman. Nancy, which is the bear. So Nancy's spent three and a half seasons trying to keep all those Nancies in their little boxes, and no matter what she does it overlaps, and that's what life is like. Which sucks. So when Nancy sees lights out on the porch, and finds a tin-lamp light show of sailboats whirling on the walls, and goes to the porch and cuddles beside Esteban in the silence, with huge bodyguards standing outside, that's about feeling safe for five seconds.