Definition: Given a real-life anecdote of a complex ethical situation, generate a suitable title that describes the main event/root cause of the situation. Imagine that the given text is a social media post, and you have to write the title of the post, so the users can decide to read the main text or not.
Input: I haven't decided what to do here yet, and I feel like anything I do that is not outright fully supporting her in this endeavor will really upset her.

So my sister has three boys (two from a previous relationship and an infant son with her current husband), and also three stepdaughters her husband brought into the relationship. She always had said that she just wanted to have one baby that was theirs, but has not really been able to get past her gender disappointment that that one baby was, in fact, another boy.

She has always wanted a daughter. Becoming a bonus mom to three girls did not satisfy that desire. Another baby wasn't what she wanted, she wanted a daughter. So she has decided that she wants to have yet another baby but isn't willing to leave it up to chance this time--she has decided she wants to go the IVF route in order to select the sex of this next baby. She is not even remotely willing to consider that this not an entirely rational plan (let alone ethical). She is entirely unwilling to adopt because she wants to ensure she "knows the baby's history" and has what seems to be excessive concerns about behavior issues coming from an adoptive situation.

She's asked me to come along to the consultation with whatever fertility doctor she's found to help her out with his plan because she's concerned that her husband would not pay enough attention or help her ask the right questions/retain the information given.  I...am not sure that I am on board with this. I love her and will love a niece regardless of how she came into the family, but I really feel like this is not the right way to accomplish this goal of "getting her girl".

She and her husband definitely do have substantial financial means to comfortably raise an additional child and also to fund whatever method she would pursue here.  She had a pretty traumatic pregnancy and delivery with her most recent baby, right before that one they had a loss that was a girl (which she never really properly processed or grieved), and does not seem willing to believe that IVF is not just this simple thing you do once/that she might have a difficult time finding a doctor willing to go this route with her to begin with. 

So...would I be the asshole if I wasn't all-in-fully-present with my support for my sister while she tries to find a way to have a biological daughter?

not going to a fertility consultation with my sister