Is your dog not nice to your sisters dog? Or are you not sure how to meet other dogs?
People often expect their dog to get along with each dog they meet. Just like you don't like every person you meet, your dog doesn't like every dog they meet. So sometimes, they simply won't get along.
Make sure to keep following tips in mind when meeting a new dog.
Exercise both dogs
By making sure both dogs are already exercised, they will be less likely to hold any tension. This allows them to be more relaxed during the meeting
Make sure you know the other dog
Do not let your dog run up to any dog you never met before. The dog might not like this and act in order to defend himself. Be sure to talk to the owners and before giving them a chance to get to know eachother.
Don't force anything
Don't force your dog to walk up to another dog. If he doesn't like this, he might act badly because of insecurity or fear. Give your dog the opportunity to get closer at his own pace.
Take a walk together
By walking, your dog will have an easier time accepting the other dog around him. His mind will shift from focussing on the dog to focussing on the walk. He still knows the dog is around, but will start to accept him more easily.
If you both trust your dog and are sure they will listen to you when called, it's preferable to let them meet off-leash. This way they can give clear signals to eachother about how they feel. Also, they will feel more confident themselves because they are not confined to a leash.
Give them time
Much like humans, dogs might need some time to warm up to eachother. We might be checking the time every now and then, dogs do not care about the time. They will decide when they are ready to accept the other dog.
Walk in circles instead of straight at eachother
When meeting while your dog is leashed, you shouldn't approach other dogs in a straight line. Unlike humans, who walk up to eachother to shake hands or say 'hi', dogs tend to circle around eachother, showing they mean no harm.
Always meet new dogs in a neutral area. I never let my dogs meet others for the first time in my home. They will act more protective because at that moment they could think of the dog as an intruder. If you want to invite another dog into your home or garden, first take them on a walk together.