I have been invited to pidyon haben of family and it is on the yahrzeit of my mother. Can I go or not?
You may go to the Pidyon Haben. The only restriction that an person has on a parents yahrtzeit is attending a wedding, but not other simchos.
There are different opinions if it is permitted or not. From the terminology of the Rema Y:D 391-3 that a person should not go to any seuda on the yahrtziet of a parent. The Piskei Teshuvos ( ibid-8), brings others that say that this is only for a chasuna but not for other seudas mitzvas. This is also the ruling of Kitzur Shulchan Aruch 221-7, Pri Megadim O:CH 447- 9. Being that we are dealing here with a minhag, being that it is controversial if it applies or not we can be lenient. This is the ruling of a numerous poskim.