Keep Sissy on a leash around the house so when she starts to bite, push and tug at the 6 yrs old clothes, you have control of her.
First I'd like to say that your son is not the weak one and he needs to dominate Sissy. Sissy is 14 weeks old and probably thinks this is playing. Does your son get on the floor with her? Does your son play with her? Does your son pull at her or poke at her? Does he run from her? All of these behaviors can be signs to a pup to play.
To teach the pup not to do this, have her on a leash while the kids are awake. When the son comes into the room, watch them both to see what behavior goes on. When Sissy gets excited and starts the biting, etc. Tell her "no" in a firm, calm voice and make her sit. When she sits, reward her. Use positive reinforcement and reward her when she does what you want her to do.
Give her alternatives to biting on your son. She is teething and needs something else to chew, i.e. ropes, kongs with treats, ice cubes with treats in the middle, frozen wash cloths, nylabones, stuff animals, etc.
Has she had OB training? Is this your first puppy? You want to show Sissy what you want her to do with consistent training. What kind of mastiff is she?
Please see below to help you with your pup:
First, you want to crate train him. Make sure you have a blanket, stuffed animal (about his size) and white noise (ticking clock or ipod with soft music) so he can sleep. He's used to cuddling with siblings.
Second: you want to keep him in a room with a family member. Mastiffs need to be near their family members.
Third: you want to start his food regimen. Are you feeding him what the breeder fed him? If yes, ok. If not, you need to do a slow transition to the new food. Mastiffs are allergic to chicken and grain in kibble. Slow transition is to feed 1/4 of new food with 3/4's of old food for 3-4 days. Transition to 1/2 new and 1/2 old foods for 3-4 days. Transition to 3/4 new and 1/4 old for 3-4 days. Transition to 100% new food. If at anytime he has diarrhea, return to former transition amounts until diarrhea stops.
Fourth: You want to keep the leash on him for a few hours each day while in the house so he gets used to it.
Fifth: Keep him away from dog areas because he hasn't had his 2nd set of shots leaving him prone to getting parvo or other illness. Keep him in your yard and place newspapers down where he will walk on the ground. This is very important!
Sixth: Start basic commands. Train for about 5 minutes per day and slowing increase time. Teach one command at a time. Once they master one command, move onto another command.
Mastiffs can be extremely stubborn and if you get frustrated with them, they will shut down. Some mastiffs do not do well with yelling.
Number one command is sit. Teach him to sit, by placing a treat in front of his head and move it to the back causing him to sit to get the treat. When he sits, tell him good sit and give him the treat.
Second command should be "focus/look" This will help you tremendously when he's over 100 lbs. Put him into sit. With a treat in your hand (let him smell it), put the treat up to your eyes and tell him to look or focus. He may only do this for about 1-2 seconds. As soon as he looks at your eyes, tell him good look or good focus and give the treat. Some mastiffs (DDBs generally) do not like to look anyone in the eyes for long because that means a challenge to them. Titan is up to 25 seconds of looking at me.
Other commands to teach is stay, come, leave it and drop it.
When you are training and if he does not do as you ask, then tell him no no no and redirect back to command in a normal voice. The only time a stern and somewhat loud NO should be used is when they are doing something that can cause harm to themselves or others.
For example, when I'm doing the look at me training with Titan. He will look at me and then his eyes will move to the left or right. I say, "no no no, look at me" and returns to the look at me.
When they do what you want them to do, get all giddy and excited and say "Yes, good look!" I clap, giggle, and sometimes do a little dance. My dog looks at me like, really woman?
Puppies should not do any heavy exercise or walking for the first year. Stairs should be maneuvered while on leash (even in the house) especially going down the stairs. Most mastiffs (DDBs especially) can be very lazy but they still need to exercise. Puppies should not be walked for more than 15-20 minutes and do your best to avoid heavy running or jumping.
Mastiffs should not be neutered/spayed until 18 months to 2 years. NO MATTER what the vet says. Early neutering can cause growth problems.
Enjoy your baby! Have lots of patience! The pup will reward you with love and loyalty!
