Between the bat incident and Dad and Lisa showing up, Matt returns to the house bringing a teenage girl (roughly Miranda's age) with him. He introduces her as Syl and explains that they met a week or so earlier in a house he thought was deserted. They've seen each other every day since and this afternoon they exchanged vows, so as far as he's considered, Syl is his wife.
Syl is very very thin (but looks like a model somehow) and has clearly been around. She's not from Howell, but has been dumped there by a guy who traveled with her and then left her there.
Miranda feels betrayed that Matt never confided in her and that he's brought back a girl roughly her own age, who he claims to love. Syl is filthy and Miranda is given the job of bringing in snow (or rain barrel water) for Syl's bathwater. But it's Matt who stays with Syl while she bathes, and it's clear from Matt's actions that he truly regards Syl as his wife. They move into the living room, even though it's the room furthest from the woodstove, and therefore coldest. Presumably Mom and Miranda in the kitchen, and now Jon alone in the dining room, have to give up one of their electric heaters each, which makes their rooms colder.
Possible things that happen with Syl: Syl is the cause of Horton's death. They've all but run out of cat food, and Syl decides Horton would be happier free, so she lets him out of the house. Miranda and Jon search for Horton and find his dead body in the woods.
Syl gets pregnant. This would be announced after Dad and Lisa arrive. Now food has to be shared even more disproportionately.
When Jon blurts out about the safe town papers, Matt (or maybe Syl) suggest that since Julie will be safe at the convent, Syl could go with Miranda and Jon, at least until the baby is born or if she isn't pregnant, until Matt can find a decent place for both of them to live. This adds even more tension to the exploding papers scene, and maybe Matt and Alex come to blows (with Miranda so confused about who she's angrier with, she can't even take sides).
When the tornado hits, Miranda is outside and sees it coming. She has a choice between going to Mrs. Nesbitt's and warning Dad and his family (which include Charlie, Alex and Julie) or going back home and warning Mom and her family (Matt, Jon and now Syl). For a moment, Miranda is paralyzed, then runs to Mrs. Nesbitt's. Possibly Syl dies in the tornado.
If Syl dies in the tornado, then the exploding paper scene has to take place before then. Unless Matt, still grieving, learns about the papers, and becomes enraged that Alex never offered them while Syl was there, since she could have been sent in lieu of either Julie or Miranda. Either way, he and Alex can get into the fight.
If Syl doesn't die, then she too is a suspect when Charlie dies. And she goes with the family at the end of the book. But I think it's stronger if she's dead by then, and we get to see Matt's devastation at her loss (and we don't water down the power of Lisa's heartless statement about how Charlie had to be stopped, by having another seemingly heartless character potentially involved).
The strongest argument against Syl dying is you don't want this to become a Bonanza style family- marry Hoss or Little Joe and inevitably end up dead. Since Charlie, an outsider dies, it might not be good for Syl, the other outsider, to be the only other character who dies. Maybe Syl doesn't die, but wanders off, having gotten what she needs from Matt and his family, while causing them great pain and division. Matt would be just as distraught and angry, especially if he senses relief on the part of Mom and maybe the others (certainly Miranda). In which case, Syl wouldn't be pregnant, but would leave after the tornado (when things are at their worst, since Dad's crew has moved back into the house). Maybe Matt tries to find her and is gone a couple of days, which increases Mom's anxiety about letting Miranda go with Dad, Julie and Alex on the trip to the convent. Maybe Matt finds Syl but she won't come back with him or let him travel with her, which would leave Matt resenting his family situation even more.
A couple of other possibilities- Jon blurts out about the safe town papers after he finds Horton's dead body- "I'm glad we're leaving you behind when we go to the safe town!"- something like that.
Another thought- bringing in Syl may eliminate the need for the whole bat incident.
Matt finds Syl after she runs away and brings her back to the family, where she stays and goes with them at the end.
Miranda to Julie, "How would you feel if your brother suddenly brought home a girl and said she was his wife."
Julie, "I don't know. Relieved?"
Julie about Alex, "He's done everything he's had to do, but nothing that he's wanted to do."
When Alex leaves Miranda and Dad, Miranda is a little surprised that he doesn't give her the safe home papers. In spite of everything, he's holding onto them in case Julie ever needs them.