I really feel for you - I was a stepmother for 5 years before I had children of my own and it is a difficult line to tread. I think the key question is 'how old is the child?'. If you are dealing with a pre-schooler and you are sometimes in sole charge then they are going to a certain amount of discipline - it is a pre-schoolers job to test the boundaries and an adults job to show them where the boundaries are. If your boyfirend's child is not with him full-time then he too may find it hard to be firm because he wants the child to have fun when they are with him. All you can do is talk to him about how he wants things to be and try to keep things in line with his wishes.

Continued from answer below.... If the child is older than 5 then they already know what is naughty and what is not, and if they misbehave they are doing it to make a point ("I'm unhappy" or "you're not my mum"). However old the child, I do recommend 'How to talk so kids will listen and listen so kids will talk' - a very good book. It is definitely best to back off whenever possible - pick your battles and try to follow the same guidelines for behaviour as the child's mum and dad (even if you don't always agree).

My advice would be to leave the disciplining to your boyfriend and not get involved. However, that doesn't mean accepting bad behavior. When he or she is behaving badly gently point it out to him for him to sort out. Men are generally unaware so a gentle "jimmy is peeing in the sink can you sort it out" will promt him dealing with it. Build the relationship up slowly with the child. My daughter adores her stepfather but he took it slowly and sat back and let her learn to love him.

My family always followed the rule that says "if you're looking after my kids, you're in charge of them", and that includes tellings off where necessary. Then again, I guess we were lucky in that we knew they wouldn't dream of beating the kids senseless! I think it's best to have a good talk with your boyfriend and establish the rules up front. Kids are very good at sniffing out vulnerability and if you're the weak link in the chain, you can guarantee he or she will capitalise on it! Explain that you don't want carte blanche to smack the child, but you do want his back up where scolding bad behaviour is concerned. Good luck, it's a sensitive issue, but manageable with a bit of common sense.

Your priority should be to provide a caring, safe environment for this child in collaboration with your boyfriend. A child who feels loved and secure does not need 'disciplining' very often. When they do need to be told off, consistency, fairness and and humour go a very long way. Talk to your boyfriend about this, be open and honest. Make sure you are ready to take on this responsibility before you start.