Sorry to hear about your dilemma. I know it must be quite disheartening. Here is one view on the matter.
A lot hinges on your true feelings for the faith. Because from what I have seen you will need really strong faith to face the ups and downs that may arise from this relationship. Firstly you are very likely to face some resentment from certain family/community members, but there will certainly be others who wont take such a narrow minded view.
If in your heart of hearts, you can truly say you are Sikh of your own choice and haven't converted for other reasons, then in my view the question of you marrying another Sikh isn't an issue. What you are probably encountering is deeply entrenched Panjabi culture more than Sikhism. If you're from a Turkish background you may have some insight into conservative attitudes towards cross cultural marriages from your own experiences.
To be exact, my question is: would the kids of my friends' sisters/brothers not find marriage offers from good Sikh families, because there is a white Sikh in the family?
I don't know where you are but I think the Sikh diaspora community is likely to be different to what it is now in twenty years. In my own family there are two young children (the generation beneath mine) who are of mixed race. This hasn't stopped marriages taking place within the wider community by immediate members of the clan into "good Sikh families", whatever that means.
In all reality, the question you have to honestly ask yourself is whether you yourself are prepared to face a period of hostility that is pretty much inevitable, until you become accepted in the family/community of your friend. This may take some time and be quite unpleasant at times. The biggest fear Panjabis have is the loss of culture from inter-racial marriages, this is not entirely unfounded from my experiences.
That being said. I think it is unlikely, that some family - twenty odd years from now - is going to look down on someone because their cousins were mixed race Sikhs
, who spoke Panjabi and were culturally/religiously aware - this doesn't correspond to my experiences of the community. But you could be dealing with an ultra conservative/traditional family.