Probably the hardest and most stressful part of preparing for a photo session is deciding what everyone should wear. This short article isn't going to completely solve your problem (sorry), but it should at least help point you in the right direction.
Wear what feels comfortable - This session is about you (and your family, spouse, etc...). Don't feel like you have to dress in an entirely different style that you aren't comfortable with just because that style looked amazing on Pinterest. Dress in what feels natural to you.
No loud patterns or colors - The session should be more about you and not the clothes you're wearing. Stay away from ultra bright colors, and loud patterns.
Subtle style - While you want to be stylish, remember that styles change. The edgier the style you choose for your session, the quicker the photos will look dated. Instead, consider classic, subtle styles.
Complimentary colors & styles - Having everyone wear the same outfit or color is mostly out of fashion these days. Instead, you should look to have everyone wear colors and styles that are complimentary to each other. See some examples in the links below.
Start with one person - Once you pick an overall scheme, then start selecting outfits by selecting an outfit for the most difficult person to dress and then picking other outfits around that person.
Prepare - Don't plan to decide on outfits as you're getting dressed for the session. If you leave the decision until last minute, it's going to be stressful and not very well thought out. Plus, you won't have the time to purchase anything if you need it. (If you're reading this article, I suspect you don't have this problem). I suggest you plan out the wardrobe a few days in advance. Make sure you take weather forecasts into account.
Dress appropriately for the weather - You would think this one seems pretty obvious, but I can't even count the number of times parents show up to a cold session with their kid dressed in a "super cute" summer outfit. Not surprisingly, those kids end up cold and miserable and it shows in the photos. If it's cold out, be sure to dress accordingly. In the Philadelphia area, weather can be drastically different from day to day. Be prepared to pivot if the weather changes. Layers help, that way we can always add and remove when necessary.
Family & Children
Here is a great Pinterest board with some inspiration for attire for family and children sessions
Here is a great Pinterest board for some engagement style inspiration. Take note that in most of the examples, the attire is relatively subtle, and the photography is more about the couple.
Your options for newborn attire are: naked, diaper, an outfit, and a swaddle. It's really your choice on how we dress the baby. We can stop for a few outfit changes. Just keep in mind that there is a pretty high probability of an accident with a naked baby. If you want naked baby shots, be prepared to clean up and change everyone. Whatever you choose, just try to keep colors and patterns light and simple.
For everyone else involved (mom, dad, siblings, dog, etc...), follow the tips in the general section above. The most important thing is to keep things simple. The baby should be the center of attention in the shoot (and your lives for the foreseeable future). Sometimes patterns and loud colored clothing tends to distract. Try and stay away from bold colors and patterns (like gingham, stripes, etc...). Also, be mindful that it usually takes most women a bit of time to lose the baby weight. Shirts, sweaters, and dresses with a bit more room in them can be more complimentary. Layering can also help here.
This is probably the one time in a woman's life when the goal is to show off your (rather large) belly. Normally (in non-maternity photos), solid dark colors are great because they can be slimming and hide detail. For the opposite reason, prints and lighter colors tend to work better for maternity sessions. Sometimes patterned material works well to highlight the shape nicely (here is an example). Also to show off your shape a bit more, try to select something that's a more fitted style.
Keep things simple and elegant. Stay away from patterns and colors that are too loud as you want the session to be about you and not your distracting clothes.
Here is a Pinterest board for some maternity style inspiration.
This site has a very comprehensive guide on style for photography. It's a long article, but it's a good read and there are lots of good examples.