I don't think it sounds stupid...........love does not pick a good age to appear. The reality is it sounds like you really care for this girl, as far as letting her down only you can prevent that.
A relationship isn't about just running around being in love, it takes a great deal of hard work and morals. In life we always run into problems and often those problems cause us to second guess where we are in life and sometimes we make big mistakes out of fear and hurt those we love. The key is instead of allowing your fear to take over you when there is a problem you must look at the problem for what it is and solve it.
In this case your problem is two fold:
1) She is pregnant with another mans child...........now I am a firm believer it takes more then sperm to be a father. In fact I am currently raising a step son myself so I can tell you with 100% confidence that you can be this child's father if you want to be. A bond with a child does not start at conception, it starts when a man makes the decision to raise the child and be a father to the child.
Now I am not going to tell you it will always be easy, the reality is there are separate hurdles that come with raising a step child. But if you push though and treat the child as if it were your own these can iron themselves out quickly. Not to even mention the lesson you will be teaching this child about responsibility love and kindness.
2) Your family, now this is a bigger problem. The fact is they are going to worry about you......you are their child and they want what is best for you. They know that you and this girl have had problems in the past and the reality is you both are VERY young add a baby to that and its enough to scare the crap out of any mother or father!
Try looking at it from there position........they are not giving you grief because they like to, they are honestly just concerned about what is in the best interest of their child. Which is 100% normal and natural.
Your job is to sit down and give some real thought to this whole situation. So when you talk to your parents you can explain to them about how you feel and what you want to do. Let them know how much you love this girl and while you know you are young the reality is you want to be with her.
I would also strongly caution you to take this slow.........while you can go into this with the best of intentions that does not mean she is willing to work as hard as you are. I am not saying that she isnt.......just that it is a possibility. For that reason I would suggest you both take it slow. Don't jump into living with each other, or marriage. Continue dating and being with each other.
A ready made family can sometimes be hard for both people.......and at this time she is going to be VERY emotional due to her hormones so the last thing you want to do is add any additional stress to the relationship which could cause a blow out. You can be with her and be a father to this child even without living together or being married.........it just takes a little more work. And in the end you want to go into this with both eyes open and prepared.