I don't have a baby, but a have a DS who will be 7 in a month and a disabled 4 year old DD, who physically functions like a newborn. The truth is, I DO have to divide my time a lot! There are times I stick my DD in front of the ipad to watch a video and times DS gets ignored while I tend to DD. I do try to avoid that. Sometimes I just hold DD on my lap while I play a board game with DS, or while we do schoolwork.
They can't play like typical siblings. I always hear how great it is to have more than one, that way they can entertain each other...not so with my kids. If DD needs attention and I'm cooking, I'll tell DS to go talk with her or play with her for a minute and he'll either ignore me, or walk in, jiggle her hand for a second, then walk back out. He doesn't "get" how to help his sister yet. I'm hoping it will get better as he gets older though, as she's not going to progress developmentally and I could use the help!
One thing I've been doing, very recently, is I've started having DS bathe himself while I put DD to bed. I can hear him in the bathroom, which is right next door to her room, and it's getting 2 things done at once. He mostly just needs help washing his hair now.
Mostly though, I encourage DS to play by himself and he has one friend that I allow over at pretty much anytime (they play well together and don't destroy the house). DD needs me more, physically, and DS is old enough to entertain himself. I'm *available* to him, but I decided long ago I'm not my kid's playmate. I'll play a board game, read to him, do crafts, etc, but I don't get on the floor and play for hours. My parents never did that with me and I turned out fine. LOL
He's not yet capable of making his own food and drinks, so I keep some single serve stuff like chocolate milks and applesauce cups in the fridge so if I'm busy he can grab one until I can get up to get him a proper meal or snack. There are times DD has issues that cause her to scream, much like a colicky baby would, for hours at a time (although we got to the point we started knocking her out with meds, so more like 30 minutes at a time, so I do have that advantage over those with a truly colicky baby). Those are the worst times though because I'm not even *mentally* available to DS...we get through it though.