How does that work?
The wearer treats her sorcerer level as 4 higher than normal for the purpose of determining what bloodline powers she can use and their effects.
So, you wear the robe, your sorcerer level is considered 4 levels higher for bloodline effects.
Of course, since the stitching has to match your bloodline or your family tree, so it will have to be something you either make yourself, or have made specifically for you. But other than that, its a great item.
Some GMs might make it only work for one bloodline, and might allow the robe to augment two bloodlines for a 40,000 GP cost (16k for the first bloodline and 1.5 x 16k for the second bloodline enchantment on the same item), but the RAW doesn't support that. Technically, one could have the crossblooded archetype and have Arcane Heritage feats all bumped up by one robe, which seems a bit steep. Your GM will probably have a talk with you about that one.
The thing is that the crossblooded archetype comes with severe drawbacks. An item that promotes bloodline powers does not grant the crossblooded much more power than a single-blooded Sorcerer because after the Bloodline Arcana, they still only get one bloodline power at 3rd, 9th, 15th, and 20th.
Arcane Heritage feats aren't really better, because the level requirements for you to take anything past the first Arcane Heritage Feat is 11. You only gain bloodline abilities as you take the feats, so it isn't bad there either.
Anyway. All told, it works well, but its not as near an underpriced item as, say, the Boots of Speed. But it is a great item for any sorcerer or multi-classed sorcerer.
The point is if it works for both bloodlines (and i don't say that it doesn't) then at a certain level you have both powers and when you level you lose one of those powers as you select it.
You have to choose between your powers when you level up, you can't have two at the same time. A Draconic/Abyssal Crossblooded Sorcerer must choose between his boost to strength and his Breath Weapon. There is no use of this item that allows you to have both. It makes you a higher level character in regards to your bloodlines, it does not allow you to do something a higher level character could not do.
You have to choose between your powers when you level up, you can't have two at the same time. A Draconic/Abyssal Crossblooded Sorcerer must choose between his boost to strength and his Breath Weapon. There is no use of this item that allows you to have both. It makes you a higher level character in regards to your bloodlines, it does not allow you to do something a higher level character could not do.
But robes of arcane heritage can give you powers that you don't have, for example a 5th level abyssal bloodline sorcerer with robe of arcane heritage gets the +2 to STR.
About the number 2, so i can take it off, re-don it and select different powers?
Yeah, as a cross-blooded sorcerer, you have to pick which powers you want at a given level. A cross-blooded sorcerer with the robe in question would just need to decide which power he wanted 4 levels earlier than normal.
Definitely a great investment, though. I'll have to look into it for my own character.
About the number 2, so i can take it off, re-don it and select different powers?
I don't think that is possible, but I would let a first time player try out the various powers, but I wouldn't let him switch a whole lot. No more than once per day. Because most 9th level abilities are once per day or minute per level abilities, I would recommend that even if you allowed them to switch, that any use of the abilities of one would diminish the other as well.
So there isn't an answer to that, that's what i was searching for.
There's a number of precedents where Paizo developers have disallowed the "take it off, put it on, make a different choice" property of many magic items. Take the headband of intelligence, for example: They specifically changed it from v3.5 D&D to Pathfinder so that the skills it gave you were set.