Babies generally thrive on routine and it sounds like yours really does. Its not a bad thing at all. DS would only sleep for half an hour exactly 3 times a day from the age of about 3 months up until 11 months. You could actually set your watch by him, half an hour and his eyes were open. It was exhausting having a young baby that was awake so much in the day and he wanted entertaining.
DD is nearly 2 and she will ONLY sleep in the day in her cot. She won't settle in her pram, car seat anywhere excpet her cot. I am fine with this. There will be a time where she won't nap in the day anymore so I just go along with it.
Some children don't automatically go to other people either. Especially if they don't see them often. At Christmas, DH's family commented on how clingy DDwas. She had only seen some of them once in her life when she was a few weeks old and the others she barely sees so she didn't know who they were yet they expected her to come running to them.
I talked loads to mine and they both proved to be early and good talkers and DS is doing great with reading and things at school. I put a lot of that down to talking and reading to him from the very start.
Every baby is different and comparing yours to your sisters is ridiculous. They all do their own thing. Mine where incredibly laid back in personality (nothing like me whatsoever) but it didn't last with DS past 3 and DD is almost 2 and there are signs of a very firey personality.
It sounds like you are doing a great job. Your DS clearly feels very comfortable in the routine you have given him so why change it. DD gets tearful from about 4.45 because she is hungry and its coming up to tea time. She just knows thats what time it is.