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Is there a reason they need to pee on the pads? Because honestly, I and several others on this forum have had not great experiences with them. They're very similar (from a dog's perspective) to other absorbent pad-like objects (rugs, bath mats, a dropped shirt, etc), and it also confuses the issue of whether it's okay to pee inside or not. I'd ditch them entirely and focus on training the puppies to potty where the big dogs do outside. Get an enzymatic cleaner specifically for pet messes (Nature's Miracle is widely available in the US) and thoroughly clean any spots where they've peed so they don't smell it anymore, then go back to basics with a strict potty schedule and rewards for going in the right place outside.

That being said, I don't envy you the work of two puppies. They're easily four or more times the work of one. Read up on littermate syndrome and make sure they're getting individual attention now so they can function apart from each other and bond properly with the people in the household.

You don't say if they are puppies or adults, but assuming that both of them have a clean bill of health from the vet, and don't have urinary infections, then the basic rules apply. For starters, ditch the pads, and train them to go outside. Take them out as soon as they wake up, after they've eaten, after they've had a big drink of water, after they've been playing for a while, and every 20 minutes or so just because.

If you aren't around to keep an eye on them, put them in separate crates that are just big enough for them to stand, sit, and lay down in comfortably. If it's too large, they will sleep in one corner and potty in another. If they pee on bedding, then no bedding in the crates (there are solid, waterproof pads if you want).

He's not peeing for attention, and he only pees on the floor when you're not around because he's learned that peeing when you are around makes you mad. The "guilty face" and tucked, wagging tail are both what are called appeasement gestures, and are basically him asking you to please not yell at him.

So, stop using the pads, take them outside about a million times a day, and crate (with no bedding to pee on) if you can't be watching them.

It's obvious you want the best for these dogs, even if you have some hurdles to get through! That's the important bit.

I will say that wandering the same house every day isn't any more physically or mentally stimulating than it would be for us. There's a reason walks are such a big thing with dogs - they're as much or more about exposing the dog to new smells, sights, and sounds and getting their brain working than they are about exercise. I would at the very least invest in buying or making food toys and incorporating them into day to day life. There's a great Facebook group about canine enrichment here, with lots of ideas about how to use cheap things around the house to keep your dog active and encourage them to participate in natural behaviors like sniffing and foraging. It can be a lot of fun, and if you can show your siblings that, hopefully you can encourage them to keep it up when you're not home.

There's a lot of evidence that, like with humans, this kind of enrichment and mental stimulation slows down mental decline, and might even help keep a dog more limber and mobile longer. It's a powerful tool in caring for all of our dogs' needs, not just the physical ones. And, of course, it helps relieve boredom, as bored dogs are more likely to develop behavioral problems.