There may not be much significance to the dream, as in your subconscious mind, the various family relationships can blend easily into one another. Identifying your neice as an "in-law" emphasizes the connection is tied to marriage. Your dream simply might have exaggerated the marriage idea. However, any interpretation depends on what actually happened in the dream and on any emotions you felt while experiencing the dream. If it all seemed fine and natural, then there is no cause for concern.
Dreams about blood can have many different meanings depending on other elements in the dream. Similarly, dreaming of one's niece could mean many different things depending on what the niece is doing in the dream. A basic principle in understanding dreams is that the dream is about the dreamer, not about other persons that appear in the dream. So the blood and the niece in this dream reflect something about the dreamer's feelings, memories or experiences.
Assuming that the niece is well at the time of the dream, it probably uses the niece as a symbol for the dreamer or some part of the dreamer's life that is not entirely what it should be. For example, if the niece has a very active social life, the dream might be showing that the dreamer needs to get out and mingle with friends more often. > Alternatively, if the niece is not particularly healthy, the dreamer's mind could be picking up on subtle clues among family members that point toward an illness not yet announced publicly.
This dream reflects the dreamer's grief and regret that it was not possible to save the father's life.
It means you are thinking about him
It most likely means you miss your father.
you miss your Dad
Assuming that you are not married, this is a wish-fulfillment dream, expressing your longing for this to be a permanent relationship.
it means that someone is veary jelous of your father and you need to save him
The dream suggests that the dreamer feels abandoned with someone else's problem.
Your niece is the daughter of your brother or sister.
This dream suggests that the dreamer recognizes the influence of the past in the friend's life and/or the dreamer's own life.
you not over him go get him back if you still love him