Heavens UniSarah, ignore all comments, even those from people who have breastfed and their los slept through- and especially those who have never done it! Even if they have done it, they're certainly no expert. Every child is different and the only person who can really be sure they're doing right is the mother (and father, forgive me but UKWIM).
My ds slept through the night of his 2 month injections, and the next 2 nights. Then he woke again. On and off till last month really, when he was 10 months, and now I can see a definite bedtime pattern. Every day is different. There's growth spurts, illness, teething etc that will change them and other nights when there's just nothing wrong, some nights they'll sleep 8 hours, some nights 14, some days 2 naps, some days none, IME no two days are the same and provided your baby is happy and healthy I think you can't go far wrong. And well dome for breastfeeding.
Maybe your SIL's children appear to be great sleepers, but where are their faults? Food? Manners? These are the things she won't tell you about, but will quickly preach about where her children shine.
Not sure if what I said makes sense (I have a habit of rambling...) but I just mean, don't be put under pressure from anyone, you're doing so well and you should be very proud.
Little white lies are great sometimes, no-one ever need know the truth, esp. if it means you don't have to listen to their "advice"