Hello there!
Would love to see some pics of your lovely pups
What your bitch is doing IS normal but you need to gently encourage her out of it as it could well escalate as she gets older - particularly if she's living with another dog that close in age, as it's likely to make them more competitive. If it was me, I'd try and avoid triggers - 'high value' things like bones often cause problems between dogs, so don't give them to the dogs when they are together - let them have treats like that but in separate rooms and feed separately if necessary. I'd teach her to 'swap' things so she starts to relax with the idea of giving something up. Offer something that's higher value to her than the thing she has and introduce a 'leave it' command. For example, say she had a favorite ball but loves a bit of roast chicken, let her play with the ball for a bit then ask her to leave it and offer the roast chicken. When she drops the ball praise her like mad and give her the roast chicken and repeat with different objects on a regular basis.
It's hard work having two pups so close in age and not usually recommended - you'll need to do lots of individual training sessions with the dogs if you want them to focus on you rather than each other - have you been to any puppy training classes or clicker training classes?
With regards to her guarding you or your partner from the other pup - don't let her. If she growls at the other dog when you are petting her, walk away and ignore both of them - you don't want her to be rewarded for growling by getting more attention from you. If she growls, she looses the attention and if necessary make time for individual cuddles with them for a while.
It sounds like she's just reaching the age where some dogs flex their muscles a little and she just needs situations managing and consistent guidance from you for a few months to show her how to behave and fit in with everyone.. nipping it in the bud now will be a lot easier than letting these behaviours become too established. Rewarding her for good behavior is key in resolving this so try and work out ways you can do this!
I'm sure you'll get lots more good suggestions shortly, but there are also lots of useful books listed at the top of the behavior board that you might like to read
Good luck!
