Yes you are their daughter and you love them both but that doesn't give you a position in their marriage.
They have always had emotional lives that are separate from you.
My children certainly don't know, or actually have any right to know, the intracacies of my relationship with their father. Our job is to parent well, part of that has been a buffer between our children and our issues or the working machinery of our marriage so there have been many things that form the context of our relationship that they don't know about, nor should they or will they.
They know that we have an overall happy marriage and sometimes we have disagreements and how we get through them.
They don't know about our sex life (apart from the bare bones of it being a loving part of a healthy adult relationship) so they have no idea that a mid life crisis lead to problems sorting out erectile dysfunction and the impact that had - do you? They don't know about the work done to recover from a breach of trust and near infidelity - do you?
I'm not intending to be rude but do you know if your mother is so renewed by these new people because your dad has had problems for years that has meant she has had her self esteem buffeted by sexual rejection or a lack of affection? That one or other has been sexually or emotionally unavailable for years? That she has always wanted to do X but children gave him a reason to demure and now it's become clear he never will support or want to do X? Not saying that's true but why would you know?
Has she felt unsupported? Has he? Has he just found the plans they made together that got them through night wakings and overtime mean nothing to your mother?
You may feel it is appropriate to tell your mum you are proud of her achievements but that she is being tactless or you, you not your dad, are finding her ebullient reports trying but further than that you don't have the information to give you the full picture.
Your mother might be treating your father poorly, taking easy domesticity for granted and has had her head turned by a new set of opportunities, she may be about to behave very badly. Equally your father may have taken her for granted and is passive aggressive because she is no longer within a comfortable sphere of wife and mother and her independence is threatening.
It could be nothing more sinister on either part than reaching an age of renewed freedom, adult children, time for themselves, better financial position and realisation they want different things or are not the people who got through child rearing together anymore. It happens.
But none of that is anything to do with you.
It often seems that the outsider sees most of the game and how well you know your parents cements this but the spadework of marriage is mostly done behind the scenes, and while you know your parents very well that's different from knowing them as two people.
You are not their mediator or counsellor, you have bias and they don't actually owe it to you to continue a marriage that isn't working for them. Or to include you in redefining the boundaries of their marriage for the future.
You have all the rights in the world to tell your mum how her behaviour makes you feel and your dad how his makes you feel and could they stop in front of you. They can choose to reflect on this and do or not do it. But be aware if you meddle in things you don't understand and don't have a right to be then it may have a detrimental effect on your relationship with either parent in the future.
You might cause problems by going off half cocked because this could be entirely nothing of concern. If your mum has done nothing but your dad is jealous of her achievements and being petulant and you steam in to tell her to buck up - well, how long would it take you to forgive such hurtful behaviour? Punish her after any crime perhaps.
Nurture your relationship with each of them but you aren't the guardian of their relationship with each other. They've been adults for longer than you, they've known each other longer than you, they probably know what's going on without your help.