I'm in my late twenties with a 5 year old daughter and I'm currently expecting another daughter in April. My husband is amazing but he has a son that is 12 years old from a previous relationship. His son has ADHD but he takes medication for it on school days but my husband has custody of his son every weekend and his mother doesn't allow him to be medicated on the weekend when he's with us. (My husband doesn't pick him up every weekend though because his mom lives over an hour away) at first when we started to date (I was 22. He was 30) I was okay not thrilled that he had a child but I was okay with it because I really liked my husband. Then we had our daughter three years into the relationship that's when it took a toll for the worse. His son and my daughter have the same exact birthday (my husband loves it, I hate it) his son has issues that my husband tends to overlook. I caught his son trying to cut my then 4 month old baby leg with a CD!! I was in the room a foot away and he was trying to cut her leg under a blanket. He was 8 at the time, old enough to know better. Ever since then he was not allowed to be alone with my daughter. He was never allowed to hold her or be alone with her without an adult around. I realized then, that was a warning sign of his future. His mother who is a gold digging, lazy, waste of space (her family is well off and takes care of her and her kids and she works for their family company but still ask for us to help with her bills even though she has many designer purses, goes on vacations constantly, expensive car)  she has another child with another guy as well. My husband BM is constantly calling my husband and complaining that their son is abusing or hitting her other child. (Ex: throws a stapler at his head, kicks, punches, cusses at him) it's exhausting when she's calling his phone all the time. I tell my husband that she makes ANY excuse  to talk to my husband. He thinks I'm being paranoid but I'm not. We recently found out that his baby mama is newly divorced from her then husband who she has another child with because she chose to cheat on him with another guy. (Her ex husband is a great guy btw) so she clearly isn't the smartest person. When we do have his son, all he does is play video games all weekend. (Which is fine by me bc I don't want anyone to know he exist) I feel like an awful person bc some of my friends and coworkers don't even know he exists. I don't talk about him or bring him out in public. I've been with my husband for 8+ years and his son has never once met my family. I've kept them away because his son is disrespectful, lazy and entitled. His son also tells his mother EVERYTHING. We've told him multiple times she does not need to know anything but yet when she talk to my husband she'll bring stuff up and she'll tell us that their son told her. It's incredibly frustrating, I don't even feel comfortable in my own home when he's around. My husband knows I don't like his son, I've never told him straight up but he knows. When his son is around for the weekend we usually just exchange about 10 words to each other MAX. I keep things minimum to he doesn't have to report back to his mother with lies. I use to talk to him more prior to my daughter but he would make up lies about me and states that I treat him bad so now I basically ignore him. Sorry I went off topic. 
 

the point of my post was to ask for advice: 

his son is spending $10+ on school lunch everyday and not eatting it. He's making barely passing grades in school. He has many missing assignments and makes excuses for them being missing. He treats his mom poorly per his mother. And she states that she can no longer care for him and wants him to move in with us this upcoming school year (he'll be 13) moving to another school district, city, friends, etc.

my husband has always said him and his BM agreed that he would live with my husband when he became a teenager and I always brushed it off bc what kind of mother would want to allow their child to live with their father full time (i would want full custody of my kids no matter how crappy they treat me bc their MY kids) she says she can't do it anymore and she even tried to convince my husband to let him move in with us over winter break. I shut it down, for now..

we are about to have a another baby in April and she wants her son to move in with us in August.. my husband and I have NO help from friends or family. ( his BM has help from her retired parents and free child care. We don't) It's just us, my daughter is always a hand full and we're going to have a newborn and she wants to pawn her kid off when our life is about get crazy.. 

I think my husband is serious about his son moving in. I've honestly thought about leaving my husband if he does allow him to move in. I'm that miserable when his son is around. I'm honestly scared of his son as well. I lock my bedroom door at night when he stays over. I make sure he has no sharp items. It's sounds horrible but I feel like he would hurt me or my kids if given the chance. I have never voiced these concerns to my husband bc how do you say out loud "I think your child could be a murderer" 

I cannot handle going from 1 child to 3 kids. My husband even asked his BM "if he lives with us, are you going to pick him up on the weekend?" Her answer "sure" AKA NO. I refuse to have his son all year round. His son would have to be responsible for getting himself ready for school and he is incredibly irresponsible. He would most likely miss the bus, forget to lock the door, set the house on fire, let strangers in, skip school. He needs to have constant supervision at all times. My husband and I have to work so we are not able to give him that. It's not fair to me as well, if his son stays with us because his attention will go to his son, he is old enough to know better but doesn't. I need help, I will have a 5 year old and a newborn. It's not fair to me because I know I will be suffering and picking up the slack and he'll focus his attention on his son. His mom clearly just wants to be child free and let go of her responsibility which is far to me. I've even thought about letting his son stay over for the whole summer(starting next summer) just to avoid letting him leave with us all year round. Which makes more sense to me bc he'll stay home all summer and we can monitor him with the house alarm, etc so he won't need to leave the house. (Last resort you're offer this, i haven't offered this idea to my husband) 

please help, how do I get out of letting my stepson move in with us? Thanks...