16 year old sister or my 24 year old cousin as Maid of Honor

I'm 24 years old also. Okay so I know this looks like a no brainer right? Well... it's not. You see, you would think my cousin is more mature.... she's not. My sister is much more mature and understands a lot more than my cousin. I trust my sister WAY more than my cousin. I love them both to death so both of them of course will be in the wedding.. but as much as I want my sister to be the Maid of Honor, my mom thinks it's a weird idea and that's she's too young and since she's only a sophomore in high school, she wouldn't be able to help out with the wedding financially, but my fiance and I are handling everything on our own so I'm not really sure why should would need to be buyung stuff for our wedding. I'm still researching as to what the true role of a Maid of Honor is. I'm new to being engaged lol I had no clue how detailed and stressful wedding planning would be. I never grew up envisioning my wedding. Very new stuff for me. So.. can any help me or even ask more questions?