When is wrong to tell the truth?

Hi, i [35f] and I'm marrying a [38m] with three young ones, twins 11 and youngest 7. I don't like to lie to anyone let alone children. I feel like telling them the truth and letting their young minds calculate what is going on in their lives for themselves. With that being said their mother is lying to them about my husband and myself. Telling them that they shouldn't want to be around us because we can't afford this and that and we live in a trailer. Let's give some back story.....I use to work as a CNA at a hospital but recently had to quit because I have had health issues. I had a 3 bedroom 2 bath double wide with a large yard. I payed for that home until the end of the term of the lease and moved in with my fiance. He has a 3 bedroom single wide with a roommate. I've had to sleep on the couch when the kids are here. I don't care where I sleep at this point because I have no job and no where place to sleep. That's not the problem....the problem is that one day my fiance ex decided to attack me and my family calling myself and family drug addicts. My brother has had an issue with drugs and has sold drugs to support that habit. Thank God he has been clean for over 5 years and has a great job. I've never had that problem but I had him living with me in my home with me in the trailer. To me helping others, family, or not if they show progress they deserve that help. So I didn't think otherwise to help him have a roof over his head without persecution. Fast forward to now my fiancé's ex is trying to take his kids away from him because she is telling her attorney that I am a drug addict because I lived with my brother. But she also telling him that I'm lying to their kids about her. This isn't true...one day they come me and I was upset and they asked me why I was upset. I told them that their mother wasn't being very nice to me. She had infacted called my family trash. I didn't tell them to think anything of their mother, I just said that she made me upset for being mean. A little while that day the older twin went through my phone because she likes to research stuff and look at my photos of my familes farm. She had seen that I had screenshoted what her mother had said to me. My question is what do I do. I try to not say anything at all. Now they ask me whats wrong and I just change the subject. What do I do....how do I handle this....because now she is trying to take the kids away from my fiance without a court order. He hasn't seen them for over a week she only let's him see them every other Sunday now. Idk what to do 💔 but as per their court agreement he gets them half the time. He is so hurt and she has sent an email telling him he only gets them every other Sunday because of me. I'm feel so guilty but at the same time.....I don't feel like I did anything wrong. HELP!!!!!

PS: I'm sorry if my post is confusing my health hasn't been helping me with articulating things properly. Thank God for auto correct 🙏 🙌