It's just an emotional style of relating to each other-- yes sometimes there can be cattiness, or a 'nervous' energy around whether you feel like expressing your truth to the person. It's okay to be shy about things or put it off for later - but make sure you return to it. Don't just let it wane into nothing.My experience (both in a sibling, platonic and romantic/intimate situation) has been the over-arching topic of avoidance until it's not. Sometimes the avoidance can be left too long and it boils into unresolved issues (or worse, just exited out of) but for the most part even if there are 'arguments', if things do get discussed then it's not an issue later.
Cancer wants connection very strong, but it's also a here and there energy. It moves as it's felt (cardinal) so sometimes in the 'heat' of an emotion (mars) it can be the wrong time to confront something. BIG boundaries theme ... Learn to COMMUNICATE that you need space or time to calm down or feel what you feel before discussing tougher topics.
Don't avoid expression because you think an emotion doesn't make sense or seems 'too much'. Just be honest -- Cancer might want to hide in the shadow like the Moon in it's new phase, or be a little heavy like it's full phase. Just feel it out, and remember that love should be the undercurrent of everything.