@vimalv,
First, her daughter is correct. You won't be taking her father's place. And you should not try to do so. You may want to sit down with her and your girlfriend and talk to her. Make sure she hears you say that you would never try to take her dad's place. If you love her mother, then tell her (the daughter) that you do. Make sure her mother tells her how she feels about you. This might go a long way toward building a relationship between the two of you.
You didn't say in what way religion is a barrier, so I have nothing to say on that except that you and the girlfriend should come to terms on that BEFORE advancing to the point of marriage. If not, you'll only be asking for trouble down the road.
As for your girlfriend's extended family, they will get over it. If the two of you remain together long term, they will accept you. Just don't hold it against them that they were at first hesitant to accept you. They may well have liked her deceased husband a great deal and they don't have the benefit of knowing you as well as your girlfriend has come to know you. So give them a break. They will come around.