As Tato pointed out above, relatives without children of their own can contribute to the future success of their other relatives' children. Single-parents today often experience the most stress when raising their children. Even couples often find the need to ask grandparents for help in taking care of their children. If the general environment changes in such a way such that parents find they have to spend more and more time just to (for example) be able to pay the bills, then they may find that they need to recruit more and more people to help in the raising of their children. If a couple is better than a single-parent in a situation like this, one might imagine that a couple plus a gay / childless sibling is better than a couple alone.
I believe an important question to ask is: Who are our kin anyway? If you follow the idea that we all descended from Adam and Eve, then we are in fact all kin (some more distant than others, of course). Similarly, in the evolutionary sense, if our species came from common ancestors, then we are still all kin. So when discussing "kin altruism" and preserving the survival of your "selfish genes", one might say that by helping any human being, we are in fact helping our own genes survive, as opposed to, for example, helping the genes of bacteria survive.
When considering the concept of competing genes, it is important to look at the scale of competition. If it's individual competition, then you'd screw anyone in your family, even your children, to favor your clones. If it's family competition, then you'd screw anyone you don't consider to be part of your family, or too distant of a relative. If it's ethnic group competition, then you'd screw anyone you don't consider to be in your ethnic group. If it's species competition, then you'd screw any organism that your species can't mate with (unless that species were useful in other ways, such as domesticated chickens or cabbage). Beyond species, if your "selfish genes" were just trying to propagate themselves, then they may "predispose" you to favor primates, mammals, chordates, animals, organisms, organic matter, or whatever.
Imagine a science fiction future in which humans came in contact with an alien civilization (that we can't mate with) - perhaps they aren't even composed of biological matter, but merely mechanical beings. Would they have a history of social organization similar to ours? Perhaps and perhaps not. We may introduce to them our civilization's own past experience with monarchy, dictatorship, democracy, capitalism, communism, anarchism, social democracy, etc. Perhaps even then we still haven't worked out a very good solution. They too may introduce to us their own history of various different kinds of social organization.
What happens after this kind of memetic cross-pollination? Some of us may start to take sides with some of the aliens in fighting for one ideology, while others of us take sides with other aliens to fight for other ideologies. When this occurs, "kin altruism" no longer happens in terms of helping your genetic relatives, but instead, it happens in terms of helping your "memetic relatives" - those who hold ideas similar to your own. Some may be focused on actually attacking their memetic competition, others may be focused on merely ensuring the survival or security of their own "memetic kin", while yet more may be focused on trying to convert your "non-kin" into your "kin".