I’m not surprised that you’re confused because it is true to say both that you don’t need to go to church to be a Christian and that if you don’t go to church, you’re not a Christian. Let me explain.
If we were to narrow down what it means to be a Christian, the best place to look is at one of the people who Jesus said was a Christian. If you remember the thief on the cross next to Jesus (Luke 23:39-43), Jesus promises this man that he will be with him in paradise. This man never got to go to church, and yet he is a Christian because he trusts in Jesus. So in that sense it is true to say that you don’t have to be a church goer to be a Christian. In fact as soon as you say you “have to do something” to be a Christian you are in danger of getting the gospel message wrong.
However, the Bible has no concept of individual spirituality - that is Christianity is not something personal that has nothing to do with anyone else. That is a very modern way of thinking. We can see it in the above passage, the way the man expresses his trust in Jesus is that shifts his allegiances to Jesus’ kingdom. He sees that Jesus is the true king and joins his kingdom, he joins a group of people - those ruled by Jesus the king.
If you are a Christian, then you are a member of God’s church. The church is now your family, and they are God’s gift to us in living Christian lives. We see this in Romans 8:28-30. Those who are saved are saved so that Jesus would be the big brother of many brothers. God is in the business of making a family (have a read of Romans 8). This can also be seen in the fact that most of the instructions for Christians are done in the plural, there are very few (if any) instructions to individuals. (The Greek of the NT distinguishes between singular and plural ‘you’ English doesn’t do that anymore).
The Christian life was never meant to be lived solo, God has gifted each member of his church to serve one another, you can’t do that solo. It is near impossible to live a Christian life alone, it runs counter to everything God has done for us. However, some people can’t help but live alone. The thief on the cross could not join a church, he had no choice; but where we have a choice, we really should become a part of God’s church.
It’s like someone who gets married, but never moves in with their husband. It is true that you can be married without living together, and there may be extreme circumstances that you can think of where someone may get married and not live together (if someone is on their death bed for example). But a real marriage involves relationship. Becoming a Christian means being a part of God’s family.
So my advice for your sister-in-law is, if she can join a church near to her, she should do so. If there is nothing holding her back (and there might be legitimate reasons) she should go to church. It’s best for her and for the people who are missing out on her gifts that she may bring to that church. If there is some reason why she cannot get to or go to church then encourage her to meet regularly with other christians to read the bible and pray (after all this is what the essence of church is).