Before I get into what I want to say, I just want to say I absolutely DON'T think you are BU, and I agree with what PPs have said, and I LOVE the suggestion of feeding her the baby rice (please do this!).
My DD is 14 now, so babyhood was a while ago. My DM used to keep dropping hints about things. I think one of her big ones was her sleeping on her back (recommendation at the time was to sleep on their fronts as a way of minimising cot death. Or it could have been the other way around, can't remember now). So it was all "oh, she's SO much more comfortable on her back" it was fucking cot death, ffs.... of COURSE I was going to follow most up to date advice.
But then... this was the thing. You as a parent (now) follow the great advice you're given, you've read the most up-to-date stuff, you follow your instincts and you learn what works for your child.
And then, in a blink of an eye later, someone else close to you does it. so OF COURSE you pass on what works for you, what you've learnt, etc. and they look at you politely and say "hmm".
For me, it was "lifting". Lifting (which you do when they're transitioning out of nappies) is when you take your child out of bed in the middle of the night, put them on the toilet and say "you're on the toilet, do a wee" which they do (without waking up). Worked a TREAT for my DD. Advised against now.
Incidentally, we started weaning at 4 months, and she ate like a horse (it was my birthday when she was 5 months, and we had a very small party, and DD ate EVERYTHING. We couldn't believe it).
To be very, very clear. I'm not in any way saying you're wrong, and if what you're doing is working for you, then keep with it. But (honestly) I had a 5 month old who ate for England, and if your mum has had experience of anything similar, I can understand why she's worrying that milk alone is not enough.
BUT you are mum, and you know your kid best. I'm just saying, don't be surprised as things which are 'absolutes' to you now and which came highly recommended get dismissed for future babies, and you're left saying "but this WORKED for my child...." and every mum that comes after you will look at you as suspiciously as you're looking at your mum now.
Sorry if this thread makes me sound really, really old!