How to handle issues with StepChildren????

Hi Guys, I have been reading TAM for a long time now and never really felt the need to post but now I'm experiencing some issues in my life and know if there's a place where I can find help and good advice is here so I ask all of you to help me out. I recently met my future stepchildren, in 10 minutes of knowing each other we got along very well and do get along, so far our relationship is not a problem at all. They are 12, 10 and 7, the kicker is their behavior ( I don't have children) so I'm not sure how much of these behavior is actually normal or not, regardless I would like you guys to advice me how to handle this in a proper manner from the get go as I don't know what to do.* Me and my fiance live together and the kids spend the weekends with us, during the week they are with their mother, I don't know if this is relevant or not but my fiance had a very traumatizing marriage and divorce what definitely affected the kids thats why I really want to handle this in a proper way, the children are also in counseling and unfortunately my fiance and his ex wife still have constant issues and fights over custody issues and anything else you guys can imagine ( its a highly toxic situation).* The kids and I like I said we get along very well, in a way I even think they look up to me as I bring some peace into their highly tumultuous lives and I'm a very calm person ( I don't yell or curse at them) like their parents do and I think that gives them some peace. Now the issues, they stay with us from friday afternoon until sunday afternoon, we do their food shopping on friday morning and to feed them we spend $300 what every month makes it around $1200 just in food for them keep in mind all our other bills, it is crazy and I don't know how we will keep affording it, I can no longer do it and don't know how to handle it anymore, they don't do anything else all day but play video games, watch tv and eat but even worse than this is they refuse and don't eat anything but junk food that they will also waste and literally waste most of what they pick to eat, they will either heat it on the microwave, pour it on a glass if is a drink or open a package and then literally not eat it and throw it in the garbage. They throw stuff all over the house, if they take something out of the closet they will throw it on the floor, they will use the bathroom and not flush the toilet, leave dirty clothes, toys, dishes etc all over the house, fight constan tly and do not respect any adults, our house every sunday after they leave looks like is a war zone. I have kept strong boundaries so far and do not allow them to disrespect me, they also constantly try to tell you what to do and order you around, their mother allows this behavior and my fiance has no clue how to handle it and doesnt want to be the bad guy, I just dont know what to do about all this or how to handle it, I want to handle it well from the get go, also how to talk to my fiance about it???? I don't want him to think I don't like his kids because I do and I do like to have them over I just don't know how to handle this issues and financially is destroying me. Please guys help me.

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