I think it also depends on how far away the wedding is and how close you are to this friend. When my daughter was 6 weeks old we went to my best friend's wedding in Somerset - we drove down the night before and stayed in a hotel near the motorway so easy access. Husband ended up taking baby outside church during most of the ceremony so I could stay in there, but I fed her in car before ceremony, then went to friends house nearby for big feed, then went to reception arriving a bit later than others. Fed her during dinner, after which she proceeded to do a huge vomit all down my back and over the floor of the venue, and hence required another feed - as I recall this was just before the speeches started so caused maximum disruption! I didn't see any of the speeches, which is a shame... Thankfully I have a very forgiving friend who was aware that I might not be very attentive during the reception! We drove home after the reception and it was the longest our daughter had ever slept in one go, I think - she went about 6 hours - to the point where we started wondering if we should wake her up for some food - we didn't! I think if there are others there who will understand if you are not very wedding focused, and the friend whose wedding it is also understands (depends whether there are lots of other babies about - if you're one of the first of your friends to have a baby they will think you are making a massive fuss about nothing!!), and you can get there/to the reception/hotel relatively easily and most importantly, in your own time, then it is possible. Having your niece there is a benefit but at that age, unless your baby is particularly early, you're not that likely to want to leave it with someone else I'd suggest.