to the forums!
First of all, thank you for agreeing to take in your in-laws' budgie! I'm sure he'll be very happy under your care.
You're right to be concerned, as you probably know, some budgies get more "wild" when they have to chose between their human and bird companions, especially when the other budgie is less tame. To start off, you'll want to hand tame the other bird at least and have him be comfortable with you and your family. Even though you've known your in-laws and the other bird has been fine, you still must quarantine it from your baby bird (at least 4-5 weeks) to prevent the spread of illnesses, as budgies hide illnesses and could be sick without you realizing it. During the quarantine period, work with their bird. It's still possible to get an untame bird used to people even though it is older than a year.
Read, talk, or sing quietly to the bird a few times a day, and if it's no longer scared of you, put your hand in the cage and rest it there a few times every day where he can see it until he's no longer afraid of it. This may take a little longer since he's an older fellow. Finally, you can offer him seed or millet in your hand and when he easily eats from it, he has developed trust in you and is more likely to pick up the habits of your current baby budgie instead of changing them.
If they are introduced to each other gradually, they shouldn't fight unless the younger baby is too "hyper" for the older boy in which case they should be caged separately but can still play together. Two males tend to get along very well, so their genders aren't an issue at all.
After the quarantine period and when you've determined that the older fellow trusts you enough, put their cages in the same room for a few days and observe them carefully. You can let them out into a bird-safe room (if they both come out of their cages) and entice them with a treat, like spray millet, which they should bond over. If, however, the older male doesn't want/like to come out of his cage, you can let the younger one meet him through the cage bars and see what he thinks.
If he acts aggressive in any way, it would be best to keep their cages next to each other and observe them more. If it seems later on that they want to meet each other, you can open both their doors and let them get to know each other. Chances are, the younger one will want to climb in the cage of the older one (if he doesn't want to come out) and you should monitor this carefully as he may not appreciate intrusion into "his" space and could be aggressive.
Generally speaking, it's best to introduce them in a neutral room, so if at all possible, get them both out of their cages. If you'd like to house both of them in the bigger cage, rearrange it completely beforehand so it's like a completely new cage and neither of them will be territorial.
I hope that helps a little and I'm wishing you all the best. Please keep us posted!
It's great to have you on the forums with us and if you have any more questions, please feel free to ask as we're always happy to help
We'd love to see pictures of your feathered friends when you get a chance
Hope to see you around!