Re: your dog bites your inlaw's dog. We'd need a lot more info to tackle that one, and since this here website is mostly a TRICKs website, here is a thread i once posted in one of the dog-aggression groups i belong to, on 7 things that helped *my* dog---all dogs are different, and what helped my dog get better (not cured, but better) may not help your dog:
http://pets.groups.yahoo.com/group/k9aggression-support/message/16188
the comments below my post on that group may also be helpful to consider, too.
At any rate, dog-aggression is such a complex matter, probably beyond the scope of a trick-training board, and you might do better to post such questions over in that group which specializes in managing dog-aggression.
It is best if YOU learn all about how to manage your own dog, if you are not committed to learn up on managing these things, then you should consider hiring a dog trainer or dog behaviorist,
who uses POSITIVE ONLY methods.
I have learned a lot from Kikopup on youtube, who is FREE and has MANY youtube videos on behavior problems,
as well as "On Talking Terms with Dogs/ Calming signals" book by Turid Rugaas.
I am still learning to this day, after almost 3 years now!! It is a process!
IF YOU DECIDE TO HIRE OUTSIDE HELP----Here is a thread which may offer help in learning
HOW TO SELECT a good trainer:
http://www.dogtrickacademy.com/members/forums/threads/before-asking-for-advice.160/
(be aware absolutely ANYONE can hang out a sign "Dog Behaviorist" or "Dog Trainer", they are NOT all equal. Many trainers will be
great for helping you teach your dog
agility,
but have NO experience with DOG AGGRESSION, and even less
success stories with aggressive dog rehab,
which is almost a specialty area in my opinion.
SO DO ASK for their experience, and their success stories.
AND
ABOVE ALL, go observe a trainer in action prior to letting them anywhere NEAR your dog. NOt all
DO what they
SAY they do. I've had "positive only" trainers yank and yell at my dog.
WAYYYYYYYYYYY too many trainers and behaviorists recommend shock collars, yanking, scaring the dog, punishing the dog, etc etc,
for managing dog aggression, and that is a decision i hope you are able to say "NO" to.
Not *ALL* behaviorists are 'positive only', meaning no punishment or fear is used in training, this is a widespread misconception that *all* behaviorists avoid fear/pain in their training methods, because some WILL.
Dog aggression can be a difficult matter to learn to manage, some cases are more difficult than others.
I would not allow those 2 dogs to be together until you've learned how to manage an aggressive dog, or, determined which triggers set your dog off.-----doorways, gates, TOYS are
huge triggers for many dogs, food, etc etc. Some dogs are already over their threshold just be being
in a over-exciting locations, like Petsmart or dog parks or big parties.
It is very probable that those 2 dogs are going to need EXTENSIVE and on-going desensitization efforts if they *must* see each other, to be able to stay calm, but, this CAN BE DONE. My own gangsta dog HAS become pals with some dogs in our lives, that BUddy originally 'hated'.---------------- It CAN be done, one dog at a time, but it takes a while, isn't accomplished overnight.
and it is also possible, these 2 dogs will never ever be pals and you will have to keep them separated for life if you are unable to successfully desensitize your dog to the inlaw's dog.
Yelling at dogs who are fighting, or getting ready to fight, does absolutely nothing, btw.
The key is prevention. If a fight
does break out, water is one thing you can use to disrupt the fight in progress. Reaching in for dogs may cause injury to you.
The more minutes of his life that your dog spends fighting, the more difficult it might be to rehab him, so you want to learn how to prevent all fights.
Good luck! and do hang in there, and do not give up. YOu can help your dog become a better, happier, more-secure and calmer dog,
and *you* will
learn a lot along the way. 
