Firstly im sorry about your Mum but im sure she would think you are doing a great job without her, whenever you feel you need her just try to imagine what she would do in the situation and just do your best, she would be proud. His ex will always be in his familys life as she was once part of the family, put that to one side as its something you will never change and do try to just be yourself, you cant make people like you and the more you try the harder you will find it because people will see through you and see that you are not being yourself. His family will make their own minds up about you so the best thing you can do is be natural, it will make you happier and more relaxed and if they dont accept you then so be it, their loss. As for his daughter I would just treat her as you treat your own, if you feel you are being watched then try taking a step back whenever her dad is around it will take some of the pressure of you and give the family less to get at you for. Chin up!