DEAR DEIDRE: I AM having a passionate affair with my girlfriend’s mum.I know it is wrong but I am in love with her and I don’t want it to end. I’m 24 and my girlfriend is 22.We have been together for four years and have a son who is nine months old. Her mum is 39, single and sexy.My girlfriend started back at work part-time after maternity leave and her mum comes over to our house to look after our son.One morning my girlfriend had already left for work when I heard her mum arrive. She came upstairs and knocked on my bedroom door.She was wearing a low-cut top and skinny jeans. She looked amazing. Iwas staring at her body.She gave me a smile and I went over to her and started kissing her. At first she resisted but later she allowed me to kiss her all over.We ended up having full sex. It was awesome. She is a passionate woman. Iprefer sex with her over my girlfriend because it feels so much better.She has a contraceptive implant whereas my girlfriend always insists I use condoms.She has started looking after me in other ways too — doing my washing, ironing and preparing healthy food for me.It feels as though she loves me more than my girlfriend does. One morning my girlfriend’s brother walked in on the pair of us having sex.He was furious with me but my mother-in-law told him off for creeping up on us and she warned him not to come near the bedroom again.Iwas frightened he would tell my girlfriend but my lover assured me it would be OK and we could carry on as we are.It seems she’s right — he hasn’t told anyone so far and I doubt he will — but I worry where all this will end.DEIDRE SAYS: So you should. Your girlfriend’s brother may blurt it out at any moment and, one way or another, it’s only a matter of time before your girlfriend finds out.She’s being betrayed by the two people she should be able to trust and rely on the most.Ask yourself what sort of a woman encourages her own daughter’s partner to cheat. Tell your lover you have now realised you are both making a terrible mistake and from now on your relationship will be strictly above board.Having a baby often puts a strain on the parents’ relationship but for your son’s sake as well as yours and your girlfriend’s, make a real effort to rediscover the passion.Maybe she needs more practical help from you, for starters. My e-leaflet Sex After A Baby can help, and my Guide To Contraception will help you find a better method to suit you both.