I feel like posting my own opinions on incesty stuff.
In terms of actual, real, "we all agree that it's incest" sex between family members, I have a few different opinions.
1. Anti-rape/incest and incest support groups (not "we support incest" but "we support incest victims") and advocates of support have said that there is "no such thing as consensual incest between a parent and a child." And I agree with them, to a point. Even if a person is of legal age (which, remind you, may be as low as 16 in the USA or 12 years old or younger in other parts of the world), they may be so dependent on and influenced by their parents that the parent may easily manipulate the situation so that the child may be "okay" or even want the sexual relationship. In the end, however, the parent is the one pulling the strings. I feel (although I have no proof to back this up) that parent/child incest relationships are created by the parent because the parent has lustful urges and can easily convince the child that it is okay. The idea that a parent wants a whirlwind romantic relationship with their child just seems odd to me. So I am against parent/child incest relationships in that the parent has a hard-on and they can make their kid be okay with taking it.
On the other hand, if the child is 40 and the parent is 65, does the parent still have so much control over the child? I think perhaps once a certain point is reached for both parties, the parent/child incest relationship may not be so manipulative.
2. I'm not entirely sure about how I feel in regards to sibling incest. I mean, it's not like I think of this topic excessively. Children of young ages sexually exploring each other is common, and I do not believe it is entirely unhealthy. Sooner or later kids want to figure stuff out, and especially when they are pre-pubescent and the acts aren't really sexual, well then dandy. Perhaps the sibling incestuous relationship can also be manipulative, in the case of an older sibling with a younger one, et cetera.
We all know that sex isn't only for reproduction anymore, and I do feel that the argument "incest relationships are bad because they make genetically over-matched babies!" is a little antiquated. If brothers and sisters (and any combination thereof) want to have a romantic, or purely lustful, relationship together when they both understand what's going on, I honestly cannot say I think that's a bad thing. They're two adults, let them have at it if they want. I know some people are very black and white on this issue, and 'all incest is wrong no matter what', but I don't see it that way.
3. As far as underage incest goes, I am for the most part wholly against it. Sure, kids in high school sleep around when they're 14 or 15, and good for them I guess. But when it comes to a family structure, so much of the information about life and support you get is from your family that if you start to be manipulated or "encouraged" one way or the other, it can have life-long ramifications. Most people are also imprinted based on their early sexual experiences, so if someone has sex with a family member at a young age, this could develop in to a fetish where they desire to sexually use their own children.
I suppose it's a mixed bag for me. Some elements I am super against. Other times, it doesn't bother me.
There are always some concerns I suppose. Is it healthy for them? Would it not be better for them to have a 'normal' relationship? Do they not realize that their relationship is perverse or even abusive?
I would not say "live and let live" if a relationship is clearly abusive, damaged, or unhealthy. But I think there comes a point when the people involved are old enough and educated enough that an incestuous relationship is no longer inherently damaging.