My advice to the OP: Get used to it. It doesn't mean they don't value your opinion. It doesn't mean they're dumb. It's just a silly hurdle they somehow can't get over.
I have family members just like this. We have the same discussions over and over... "I should really get rid of Merrill Lynch ... especially after seeing that article you showed me of how it will cost me $500k over my lifetime." "I should really sit down and do my own taxes this year, do you think TurboTax/Taxact/H&RBlock software will handle it?" etc.
The thing is your sister (and mine) are probably frustrated with you (and me) for something silly you are/I am ignoring that she is saying.
Yup. I argued with my dad about his financial advisor on multiple occasions. It generally went like this:
Me: Unless you think this guy can beat the market, I don't know why you think he's worth the money.
Dad: I don't think he can beat the market, I just think if the market goes down I won't lose as much money with this guy.
Me: That is literally the definition of beating the market!
Sister and Brother-in-law: [AWKWARD SILENCE]
Anyway, his portfolio significantly underperformed (actually lost money over the last 10 years or so, yeesh), he stopped using the advisor, and he told me I was right. Neither of us was particularly happy about me being right and him being wrong, but what can you do?
People are nervous about money and they want someone else to make the decisions for them so they know they're not doing something "dumb." I think that's the main benefit most people get from their FAs - reassurance. (Expensive effing reassurance!)