I think it's just baby dependent how much they wee. But point the willy down. They get erections right from birth and if it's pointing the wrong way they will just wee straight out of the nappy.
When you potty train put a cheerio or a ping pong ball in the toilet to aim at.
Siblings - lots of "our baby" and don't big up the big sibling being so grown up as it will be a sore point for them. Keep referring to her as "my baby" too. You could do "My baby girl" and "My baby boy"?
Try not to get involved in their play/interaction even if you are alarmed by it as long as the baby is happy and safe. Remember that younger siblings survive their big siblings. Avoid any stories about big siblings actually injuring the baby, look for hilarious story threads on MN where everyone shares their
moments between the baby/big sibling, where the baby was absolutely fine.
Redirect overenthusiastic play attempts e.g. don't be like "No!!! You're going to hurt him!" try "Oh, see if you can tickle his feet - he will like that" Also, big up any interaction from the baby even when they are in that pre-interacting stage like "Oh, baby really loves those tickles!" "Look, he's falling asleep. He must feel safe with you."
And on the other side, you can pretend to tell the baby off when they do something like flail their arm and "hit" the bigger sibling even though you know they don't understand and can't even control their arm yet. It just helps the big one to know that they aren't being treated differently.
Also along these lines if you are doing something for the big one and the baby is crying then talk to the baby (for the sibling's sake mainly) saying "I will be with you in a minute. I am just helping big sister right now. She needs me here. Don't worry. I'll be there soon." Helps big sister see she is not the only one who needs to wait and be patient sometimes.
Slings are great. Buy a stash of one-handed activities you can do with 5yo while breastfeeding. Number/letter workbooks are good, also colouring, stickers, some board games (but beware anything that will be ruined if the baby jolts the board)
Expect a temporary regression from the 5yo in things like sleeping, eating, toileting, behaviour, independence. Don't try to fix it, just love her through it and she will come out of it fairly quickly.