I understand how you feel, I'm looking ahead at the moment myself to think about when I'm going to give up bfing 18 week old DS.
I suppose it boils down to the fact that you are your DD's mum, not your SIL or anyone else in the family, so you have to be the one to make the choice. Try not to compare yourself to anyone else.
You've done excellent getting to this stage with bfing so give yourself a pat on the back!
You've given her the best start in life, but in truth bfing is a small part of being a good mum.
There are many other ways you can show her love, nurture her, give her good food when she's weaned etc. So if you choose you stop bfing sooner than your family did, then it's no problem and won't do her any harm, just a different way of doing things.
I guess what I'm trying to say is that there is no right or wrong, so it's really just what you as the mum think is the best course of action for you and your baby.
Some women feel much more strongly about bfing than perhaps I do, and I'm sure others will be along soon to counter/reinforce what I've said.