I have to say, from personal experience, it's best to talk about these things or they just end up festering and becoming an even bigger issue.
If you don't fancy a face-to-face session, could you call her when she's gone home and say that, although you love having their company, with 4 children it's an awful lot of hard work unless your guests are prepared to help out. Also the cost of the extra food is making your budget more difficult to adhere to. So, if she wants to come every week, could you agree certain tasks that she and step-dad will do and would it be possible for her to bring certain food items with her. Alternatively, you'd be happy to see her every other weekend as your budget could probably manage that.
Difficult, I know, but I've always found that, when I've made my feelings known, the other party (usually my mum) has been completely oblivious to how I felt previously.