Okay, suitable conversation is interesting, because when the Buddha talks about the Seven suitable conditions, he mentions suitable conversation as one of them, he is also talking about wisdom development. He is talking about growing in wisdom which therefore means suitable conversation is when we are talking about the Dhamma, we are discussing human potential, human growth, Dukkha and how to end Dhukka and everything surrounding that type of wisdom.
That is basically suitable conversation that the Buddha was talking about in the Seven suitable conditions for our growth. Now, when you are with family members, how do you adapt that? One way is to try to stay out of unsuitable conversation. That is a tricky one, but that is part of Right Speech. Can you learn how to bend conversations you have with family and friends away from unsuitable conversation? So that if the conversation becomes neutral, at least that is more suitable for you than the unbeneficial conversation, the unsuitable conversation.
Otherwise, the basic definition of suitable conversation is conversation that is helping us with our growth. Everything around the Dhamma, everything around the Four Noble Truths, the Noble Eightfold Path and so on, can be considered suitable conversation.
As to how to assess what is beneficial and appropriate speech, there is a little thing that I say in the regular retreat if you remember, "you can only be as wise as you are in each present moment, but a handy little phrase to keep in mind is, 'when in doubt - don't, consider the task and ask.'" If you are unsure whether it is beneficial and appropriate, talk to your good friends, talk to your Kalyanamittas, see whether they can help you develop more Right Understanding. Otherwise do the best you can at that time, you may consider a conversation as appropriate and beneficial, try it out and see if it works, if it doesn't work you have learned something new. But if you can talk to a good friend first, that can help a great deal.