In OTL, there are speculations and unanswered questions about the relationship between Geli Raubal and Adolf Hitler. Personally, I believe they never "did anything" even though Adolf probably wanted to, either due to Geli's objection or Adolf's sexual incapacity.
So, WI the relationship worked out and Adolf was getting a piece of his niece on a regular basis? I'm assuming that if she consented to (or at least "cooperated with") the relationship, Adolf may not have been so personally controlling over her, maybe even allowing her to date other men (as cover?) and/or take the singing lessons in Vienna she wanted, thus preventing her OTL suicide in 1931.
Some thoughts: ITTL, Adolf would have never become a vegetarian or gotten involved with Eva Braun.
Presumably, Adolf would be more mentally/emotionally stable ITTL, though he would still have his awful beliefs. How would this affect the NSDAP's timeline trajectory and that of Germany?