Help with a leg lifting problem

8 to 10 years ago a stray was living in our garage.  We gained her trust at adopted her. The day we bought her in the house she had 2 puppies a male and female. We always get our animals fixed, so were unprepared for how attached we would get. You guessed it we kept them all.  
2 1/2? Years ago we found 3 Sheppard mixed puppies In the yard. After about 4 weeks of searching for there owners, we figured they were probably dumped. You guessed it we kept them. Two females and a male.  
They were all fixed as soon as it was safe to do. The boys never lifted there legs to pee until recently. All of a sudden they started to lift their legs and mark there territory. First outside, and now in the house. I don't know what triggered it. I don't know how to stop it.
I caught the older dog in the act yesterday. I grabbed him showed him the pee told him he was bad smack his rear( I didn't hurt him) and put him outside for 20 minutes. He is a Chihuahua mix, so that's a terrible punishment for him.  They have peed on the Christmas tree several times, and if a bag or box is on the floor they pee on it. We scold them. Put them outside. Clean it up with the enzyme spray that's supposed to get rid of all the smell.  We have been making sure they get let out often to go potty.  
If you have any suggestions and I would love some help. We love them, they are great dogs. They don't really like each other, but they don't fight, or be mean to each other.  It's just strange that after about 2 years they start this terrible behavior.

I can't speak to why they might start peeing inside.... but we have a male mutt who is fixed and he sometimes squats and sometimes raises his leg in order to pee. So I'm not sure the two are related?

something is shaking up their social order and making them feel obliged to show who is who. Your old dog finally isn`t the boss, maybe?
I had this happen with my dog (2 y o, castrated) when my daughter's friend came to stay for a few days. He was unsure of where he stood in the pecking order and peed on her stuff a few times. He also got whacked in the nose and thrown outside (like you said, mine would prefer to die than be away from me).

It sounds like yours are peeing on things that smell interesting (christmas tree, boxes are all things that have lots of people handling them). If it were me I'd try doing more training stuff with all of them and making sure they all get your attention. Teach a new trick that involves them all getting a treat. I clean with vinegar (or something even sharper smelling) where my dog marked, not to block the smell so much as to make it unpleasant for him to smell.

Tereza Okava wrote: I clean with vinegar (or something even sharper smelling) where my dog marked, not to block the smell so much as to make it unpleasant for him to smell.

Vinegar was the only thing that my pup would respond to when she had a bit of a furniture teething phase. I do think the enzymatic cleaner helps significantly when it comes to marking for a variety of animals but that vinegar helped discourage bad behavior in areas. We use to have a spray bottle with 50/50 water and vinegar to spritz our pup if she was misbehaving with guests. It's goofy but it worked!

This is rarely an issue on its own. Dogs tend to manage their lives to the best of their ability and it is usually our lifestyles or our demands on them that is opposite to how a pack would behave. Every situation is different, but I recommend getting and reading The Dog Listener - https://janfennellthedoglistener.com/
. She has a unique way of understanding dogs - from the dogs point of view. It has helped me with my dogs and is completely gentle and non-violent. The book has a chapter on this very subject.

Your GSD mixes are finally becoming mature.  So smelling and acting like an adult, unfortunately in boys that means they are starting to mark their territory.   So no they are NOT peeing in the house they are probably remaining totally housebroken except for peeing on plastic bags and other plastic items, and other upright surfaces that they may think of a boundaries.  Where one marks the others will mark even with the cleaner you are using.  For a bit you will have to watch them like a hawk. If you see them start to even act like they are thinking of marking inside you will need to remind them they don't do that in the house.  We use an AHH type sound to distract them and act as a mild correction then take them outside immediately or into another part of the house.  Eventually they should get the message that marking inside is not allowed. BUT it does take commitment and is a lot of work if the habit is already estabilished however it sounds like a new behavior so hopefully they will learn quickly the house belongs to you and  marking is not allowed.   IF you have them inside for weather or due to size and would rather allow them to run loose without having to watch every second you can look up belly bands and try them on your worst offenders.