I had a friend who made me feel similar to your SIL. She too made me doubt myself, but luckily I couldn't bring myself to force some routine on my baby that I didn't want and deep down, new wouldn't have been right for either of us.
First things first, anything is normal with breastfeeding. At one end of the scale you have babies who seem to easy fall into a 3/4 hoursly feeding pattern and have a big feed at those times. At the other you have babies who want to eat little and often. Grown up people are like that too! Babies, like people are all different.
The other issue is where you stand on giving the breast for comfort. At the start I was wary of this and thought I shouldn't do it, until I actually thought why and the only thing I could think of was so vauge worry about dependancy which is rubbish because the more comfort and reassurance you give IMO, the more confident and indepentant your child can become. You can give comfort through breastfeeding and babies need comfort as much as milk. Nature is a very clever thing and it has designed us to be able to give our babies both of these things! Anyway, I'm loosing my point. The point is, even if some of those feeds are for comfort, is there anything wrong with that?
8 weeks is still very early and it took me personally allot longer than that to really learn what my babie wanted. I worried about the same thing, but kept doing what I was doing and stopped watching the clock. If he hadn't fed for a while, I offered him milk. If he had and I ruled out anything else that I could help I offered him the breast. He only really fell into anything routine like when he was being weaned.
Yes, it is logical to say that you could make your baby go further between feeds and have a nice tidy, predictable 4 hourly feeding pattern in which your baby has a big feed. If you wanted, you could gradually try and and coax him into it. But, do you really want to and if so why? knew that would not have worked for my DS and I believed in giving him my breast for comfort.
I think rouines can be great, but I believe in the ones that develop over time and you develop yourself to fit around you and your baby, not just because they are 'nice and tidy'.
To reassure you, my DS fed just like you described and is a big, healthy boy now and I have no regrets for following his lead.
The 'no-cry sleep solution'by Elaine Pantly is worth a read if you are concerned about night times, but your baby is still so young, his tummy is so small - so don't worry.
I wonder how your SIL's feeding regimne of 4 hourly feeds worked on hot summer days or during growth spurts?