Hi.
There is no right or wrong in terms of what race to date. Every individual is free to make their own choices, but those choices sometimes do have consequences, as you have seen in your own family.
I'm sorry that they are giving you so much grief about your attractions. After all, no matter what people say you "should" do, you like who you like. If your mother and sister's needling is really getting you down, you may want to let them know how much their ribbing affects you. It may help.
I also sense that you're frustrated not just with your family but also with your dating situation. You said you sometimes think that "white guys only want white girls." If you've been looking to date for awhile and haven't had any luck, it might be tempting to think "What's wrong with ME?" The answer is nothing.
There are many interracial couples in the world and the numbers are increasing. But it may be harder for you to find a guy to return your interest because most people do stick with people of their own race--either because of social or family pressure, because they are exposed mostly to people of their own race in their neighborhoods, or because they perceive that they will have more in common with someone of their own race.
If you find that, in the face of this, you are frequently comparing yourself to white girls and thinking that something is lacking in you because you have not found someone to date yet, the road might be harder for you. What other people think about you and what you do is one thing, but in the end, it may be more important how you feel about yourself.
Best,