Hi OP, I BF both of mine until they were 2.5, I had similar comments to you and it did rile me at times. I never quoted any guidelines but if someone was saying to my child that they were too big, I'd just politely explain that DD doesn't really understand what you're saying, as it's something she's always done, there's been lots of changes in her short life, teething, eating, waking, talking etc but the boobs have always been there, it's a comfort to her.
I'm my experience I couldn't stop BF until I honestly felt my children could understand and I could reason with them, especially with night feeding. So, once they were two I started talking and explaining to them, settling limits like "boobie only at bedtime, only in the morning" etc then over the next few months we gradually stopped.
You could just explain it like that to your relatives; that your daughter wouldn't understand if you stopped now and she'd get too upset, but, once you can reason with her, which is usually around two, you can start to wean her off.
Anyone that says BF past a year is to meet the needs of the mother has never had a child where only the boob will do and with holding said boob would result in a needless almighty tantrum that could last a long time.
Toddlers have tantrums because mummy says no, at this age we have to say no a lot, to keep them safe, you just can't handle weaning with the same approach, like the act of BF, stopping needs to be as gentle, loving and nurturing as possible.
NB - it's easier with your second as you have an excuse, I just used to say "oh well I did it for DD, it's only fair I do it the same way for DS". Now we're expecting our third in Feb, I think people may have given up asking - fingers crossed 