But the majority of you are clearly more noble than me!
I'm not. It is a huge thing to ask. To take on the responsibility for a total stranger who may or may not have need for additional levels of extra help depending on how frail she is, or what kinds of geriatric hiccups she has to manage.
I take care of my MIL, she lives with us, there is no way in hell I would have foistered her on somebody else during a stressful event (like a flight) even when she was far far far more independent, even if she had been at a pretty healthy "balanced" point on the mood spectrum.
I know what to do if an older person loses her pills, needs the loo at short notice, says her legs can't take much more standing/walking, has an accident, feels unwell, is getting all stressed put and a bit frantic. Which is vital because above all else what somebody feeling a little lost/overwhelmed/out of their depth needs is to feel secure that another person knows what they are doing. I don't think it is reasonable to ask you provide that essential component, because it takes familiarity and practice to get to the point where you can genuinely provide it.
Additionally I see nothing wrong in you wanting to squeeze every single bit of "me time" put of this trip as possible. Women in particular suffer great social pressure to take on a caring/self sacrificing role in many respects of their lives, to an extent that generally men don't. It is fucking unfair that the one time you get something for you people are putting pressure on you to give up a short window of pure indulgence for the sake of putting somebody else ahead of your pleasure, when the chances are much of your life is already run on that basis.
Nobody would bat an eye if a bloke was not keen on a request of this nature, don't let your lack of be-willyed status help push you into something you don't feel equipped or comfortable about doing. Having a vagina does not mean you are a horrible person if you want a rare bit of "me, it's all about me for a change!" (that lasts from start to finish as planned and looked forward to), rather than taking on a caring role for the sake of several others peace of mind. I'd hazard a guess that you do enough setting aside of you wants for the sake of other people's needs on a daily basis as it is.
Apart from anything else, it is NOT necessary to impose on somebody else by asking for a pretty massive favour. Let her relatives know that some airlines let elderly passengers travel under the unaccompanied minor scheme, or if they are feeling flush, there is this.
www.flyingcompanions.com/default.aspx