Autistic people can undergo tremendous amounts of pressure to perform "normally," and the best way to help them is to respect them. Make it clear that you will listen to them. If they feel supported at home, they will communicate and adapt better, and feel happy. Children learn to understand speech by hearing others speak, and talking to an uncommunicative person will encourage them to open up (even if the conversations are fairly one-sided for now). If you know what their special interests are, start conversations about them. Read their body language as you talk to them. For example, if you ask your daughter "Did you play with your friends today?" and she squeals happily and waves her arms, this is her answer. This communication is a stepping stone and should be encouraged. Act with the assumption that your loved one can hear and understand you, even if it doesn't look like it. Treat them like they are fundamentally good and intelligent. Positive expectations can help them bloom. If your loved one is in the room, assume they can hear what you're saying. If you talk about how hard it is to live with an autistic child, they'll worry that they make your life worse by existing. Save your adult fears for when the children are out of the room. Let them know that they're autistic. This can help them have words for their experiences, and dispel any confusion that they're "broken" or "bad." Let them know that they're just different, that this is okay, and you love them for who they are.
Treat your loved one with kindness and respect. Talk to your loved one often to encourage communication. Presume competence. Be transparent about what's going on.