Congratulations :). Ok, well first of all ignore your MIL and anyone else who mentions coping/managing etc, etc, etc :).
The main difference for me has been the lack of focus on the newborn, with DS1 everything revolved around him but with DS2 he just sort of fits in with what DS1 is doing and because of that he is far more relaxed and able to just doze when he needs to rather than constantly being held and rocked to sleep. Then the second thing is that I can't relax as much or take naps when I need to because DS1 is always there. Some days that is very hard, particularly if DS2 has had a restless night but I just make sure I have a plan for the day so I'm not having to think on my feet. Usually it involves the sand pit, stories or going for walks (where your oldest might fall asleep if you have a double buggy) or a living room picnic which DS1 loves.
In a way I've just had to become more organised and more forgiving of myself when things don't go the way I planned. If nothing gets done around the house then so be it, if I have to use the tv to babysit while I sort things out then that's fine as long as it's not all day. I've taken better care of my diet this time around to ensure that I have energy rather than that sluggish feeling. The other thing I've changed which I don't like is that I have to go to bed about 9pm, otherwise I feel terrible the next day.
It doesn't seem much harder at all for me at this point other than my days are pretty full on. I feel guilty quite often that I can't give either of them the amount of attention that DS1 got but in a positive way it has taken the pressure off DS1 and he is learning to be more independent - which he has been craving. Probably the best change has been that I value my time more, when I get a half hour to myself I really make the most of it.
Everything will be absolutely fine, things change but they're really not permanent changes so even in a difficult patch you know it won't last forever :).